chore(ci): initial flake.nix file with dev shell

Signed-off-by: Jakub Sokołowski <jakub@status.im>

flake.nix

@@ -0,0 +1,34 @@
+{
+  description = "nim-libp2p dev shell flake";
+
+  nixConfig = {
+    extra-substituters = [ "https://nix-cache.status.im/" ];
+    extra-trusted-public-keys = [ "nix-cache.status.im-1:x/93lOfLU+duPplwMSBR+OlY4+mo+dCN7n0mr4oPwgY=" ];
+  };
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      stableSystems = [
+        "x86_64-linux" "aarch64-linux" "armv7a-linux"
+        "x86_64-darwin" "aarch64-darwin"
+        "x86_64-windows"
+      ];
+      forEach = nixpkgs.lib.genAttrs;
+      forAllSystems = forEach stableSystems;
+      pkgsFor = forEach stableSystems (
+        system: import nixpkgs { inherit system; }
+      );
+    in rec {
+      devShells = forAllSystems (system: {
+        default = pkgsFor.${system}.mkShell {
+          nativeBuildInputs = with pkgsFor.${system}; [
+            nim-2_2 nimble openssl.dev
+          ];
+        };
+      });
+    };
+}