'bundle show' output in linguist.LoadColors can contain garbage

It seems Bundler's bundle executable prints warnings to stdout instead of stderr. This makes it unsuitable for scripting the way we do in https://gitlab.com/gitlab-org/gitaly/blob/66b59f65eb4434bdb943807ff2df0af72f468387/internal/linguist/linguist.go#L58-66

We need to find a more reliable way to get the path to the linguist gem.