ruby-modules/gem: fix path to git checkout

In case of the gem type 'git', nix-bundle-install.rb was called with
wrong path to the git checkout.

${src} does contain the sources, but not the newly generated .git dir,
which is created in the buildPhase of pkgs/development/ruby-modules/gem/default.nix

In some rare cases, this .git dir is needed at installPhase.

pkgs/development/ruby-modules/gem/default.nix

@@ -179,7 +179,7 @@ stdenv.mkDerivation ((builtins.removeAttrs attrs ["source"]) // {
       '${version}' \
       '${lib.escapeShellArgs buildFlags}' \
       '${attrs.source.url}' \
-      '${src}' \
+      '.' \
       '${attrs.source.rev}'
     ''}