Allow flake to be consumed, and nix build .#rln (#340)

I had been trying to consume zerokit (specifically rln on x86_64), to
build libwaku (nwaku) and was having issues, this PR at least allows a
build to occur.

```bash
$ nix flake show github:vacp2p/zerokit
error: syntax error, unexpected '=', expecting ';'
       at «github:vacp2p/zerokit/0b00c639a059a2cfde74bcf68fdf75db3b6898a4»/flake.nix:36:25:
           35|
           36|         rln-linux-arm64 = buildRln {
             |                         ^
           37|           target-platform = "aarch64-multiplatform";
```

`Cargo.lock` is required in repo for this to be possible, otherwise:
```bash
$ nix build .#rln --show-trace
warning: Git tree '/home/j/experiments/zerokit' is dirty
error:
       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:37:12:
           36|
           37|   strict = derivationStrict drvAttrs;
             |            ^
           38|

       … while evaluating derivation 'zerokit-nightly'
         whose name attribute is located at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/stdenv/generic/make-derivation.nix:336:7

       … while evaluating attribute 'cargoDeps' of derivation 'zerokit-nightly'
         at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/build-support/rust/build-rust-package/default.nix:157:5:
          156|   // {
          157|     cargoDeps = cargoDeps';
             |     ^
          158|     inherit buildAndTestSubdir;

       … while calling the 'getAttr' builtin
         at <nix/derivation-internal.nix>:50:17:
           49|     value = commonAttrs // {
           50|       outPath = builtins.getAttr outputName strict;
             |                 ^
           51|       drvPath = strict.drvPath;

       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:37:12:
           36|
           37|   strict = derivationStrict drvAttrs;
             |            ^
           38|

       … while evaluating derivation 'cargo-vendor-dir'
         whose name attribute is located at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/stdenv/generic/make-derivation.nix:336:7

       … while evaluating attribute 'buildCommand' of derivation 'cargo-vendor-dir'
         at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/build-support/trivial-builders/default.nix:59:17:
           58|         enableParallelBuilding = true;
           59|         inherit buildCommand name;
             |                 ^
           60|         passAsFile = [ "buildCommand" ]

       … while calling the 'toString' builtin
         at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/build-support/rust/import-cargo-lock.nix:264:20:
          263|
          264|     for crate in ${toString depCrates}; do
             |                    ^
          265|       # Link the crate directory, removing the output path hash from the destination.

       … while calling the 'deepSeq' builtin
         at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/build-support/rust/import-cargo-lock.nix:68:15:
           67|   # being evaluated otherwise, since there could be no git dependencies.
           68|   depCrates = builtins.deepSeq gitShaOutputHash (builtins.map mkCrate depPackages);
             |               ^
           69|

       … while calling the 'map' builtin
         at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/build-support/rust/import-cargo-lock.nix:68:50:
           67|   # being evaluated otherwise, since there could be no git dependencies.
           68|   depCrates = builtins.deepSeq gitShaOutputHash (builtins.map mkCrate depPackages);
             |                                                  ^
           69|

       … while calling the 'filter' builtin
         at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/build-support/rust/import-cargo-lock.nix:61:17:
           60|   # safely skip it.
           61|   depPackages = builtins.filter (p: p ? "source") packages;
             |                 ^
           62|

       … while calling the 'fromTOML' builtin
         at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/build-support/rust/import-cargo-lock.nix:50:20:
           49|
           50|   parsedLockFile = builtins.fromTOML lockFileContents;
             |                    ^
           51|

       … while evaluating the argument passed to builtins.fromTOML

       … while calling the 'readFile' builtin
         at /nix/store/fy7zcm8ya6p215wvrlqrl8022da6asn0-source/pkgs/build-support/rust/import-cargo-lock.nix:47:10:
           46|     if lockFile != null
           47|     then builtins.readFile lockFile
             |          ^
           48|     else args.lockFileContents;

       error: opening file '/nix/store/qh8gf0sl8znhnjwc1ksif7pwik26dsyd-source/Cargo.lock': No such file or directory
```

The PR allows for a successful build:
```bash
$ ls -R result
result:
target

result/target:
release

result/target/release:
librln.a  librln.d  librln.rlib  librln.so
```

---------

Co-authored-by: Jarrad Hope <jarrad@logos.co>
Co-authored-by: Vinh Trịnh <108657096+vinhtc27@users.noreply.github.com>

.gitignore

@@ -6,7 +6,6 @@ tmp/
 
 # Generated by Cargo will have compiled files and executables
 /target
-Cargo.lock
 
 # Generated by rln-cli
 rln-cli/database

flake.nix

@@ -31,7 +31,7 @@
         buildPackage = pkgs.callPackage ./nix/default.nix;
         buildRln = (buildPackage { src = self; project = "rln"; }).override;
       in rec {
-        rln = buildRln
+        rln = buildRln { };
 
         rln-linux-arm64 = buildRln {
           target-platform = "aarch64-multiplatform";

nix/default.nix

@@ -6,6 +6,7 @@
   release ? true,
   target-platform ? null,
   rust-target ? null,
+  features ? null,
 }:
 
 let
@@ -29,10 +30,12 @@ in rustPlatform.buildRustPackage {
   src = builtins.path { path = src; name = "zerokit"; };
 
   cargoLock = {
-    lockFile = ../Cargo.lock;
+    lockFile = src + "/Cargo.lock";
     allowBuiltinFetchGit = true;
   };
 
+  nativeBuildInputs = [ pkgs.rust-cbindgen ];
+
   doCheck = false;
 
   CARGO_HOME = "/tmp";
@@ -46,11 +49,11 @@ in rustPlatform.buildRustPackage {
   '';
 
   installPhase = ''
-    mkdir -p $out/
-    for file in $(find target -name 'librln.*' | grep -v deps/); do
-      mkdir -p $out/$(dirname $file)
-      cp -r $file $out/$file
-    done
+    set -eu
+    mkdir -p $out/lib
+    find target -type f -name 'librln.*' -not -path '*/deps/*' -exec cp -v '{}' "$out/lib/" \;
+    mkdir -p $out/include
+    cbindgen ${src}/rln -l c > "$out/include/rln.h"
   '';