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.