refactor: centralize Go version definition in flake.nix

CHANGES
*   Define `getGoVersion` function in `flake.nix`.
*   Use `getGoVersion` to set Go version consistently.
*   Pass `goVersion` explicitly into `nix/shell.nix`.
*   Remove redundant Go version definition from `shell.nix`.

flake.nix

@@ -28,6 +28,8 @@
     let
       forAllSystems = nixpkgs.lib.genAttrs (import systems);
 
+      getGoVersion = system: nixpkgs.legacyPackages.${system}.go_1_24;
+
       treefmtEval = forAllSystems (
         system:
         let
@@ -47,14 +49,14 @@
         system:
         let
           pkgs = nixpkgs.legacyPackages.${system};
-          goVersion = pkgs.go_1_24;
+          goVersion = getGoVersion system;
           goEnv = gomod2nix.legacyPackages.${system}.mkGoEnv {
             pwd = ./.;
             go = goVersion;
           };
         in
         import ./nix/shell.nix {
-          inherit pkgs goEnv;
+          inherit pkgs goEnv goVersion;
           inherit (gomod2nix.legacyPackages.${system}) gomod2nix;
         }
       );
@@ -63,7 +65,7 @@
         system:
         let
           pkgs = nixpkgs.legacyPackages.${system};
-          goVersion = pkgs.go_1_24;
+          goVersion = getGoVersion system;
         in
         {
           default = self.packages.${system}.fabric;

nix/shell.nix

@@ -2,11 +2,9 @@
   pkgs,
   gomod2nix,
   goEnv,
+  goVersion
 }:
 
-let
-  goVersion = pkgs.go_1_24;
-in
 {
   default = pkgs.mkShell {
     nativeBuildInputs = [