add buildGems to ruby passthru

This is useful for adding ruby gems that don't come with `nixpkgs`

i.e. `pkgs.ruby.withPackages (_: [ (pkgs.ruby.buildGems ./my-gemset.nix).foo ])`

pkgs/development/interpreters/ruby/default.nix

@@ -248,7 +248,7 @@ let
             inherit lib stdenv makeWrapper buildRubyGem buildEnv;
             gemConfig = defaultGemConfig;
             ruby = self;
-          }) withPackages gems;
+          }) withPackages buildGems gems;
 
         } // lib.optionalAttrs useBaseRuby {
           inherit baseRuby;