fix: use buildRubyGem in flake

Author: zah

.agents/tasks/2025/06/29-2222-nix-ci-macos

@@ -0,0 +1 @@
+Address inline comments from previous PR, ensure Nix packages can build with buildGem function missing, test packaging on macOS, run smoke tests

flake.nix

@@ -91,10 +91,11 @@
 
     packages = forEachSystem (system: let
       pkgs = import nixpkgs { inherit system; };
-      buildGem = gemdir: pkgs.rubyPackages.buildRubyGem {
-        pname = builtins.baseNameOf gemdir;
+      buildGem = gemdir: pkgs.buildRubyGem {
+        gemName = builtins.baseNameOf gemdir;
         version = builtins.readFile ./version.txt;
-        src = gemdir;
+        src = ./.;
+        sourceRoot = gemdir;
       };
     in {
       codetracer-ruby-recorder = buildGem ./gems/codetracer-ruby-recorder;