Allow customize GitLab Gemfile

What does this Merge Request do and why?

This is to implement gitlab#338469 (closed) so that we can specify that it should look for jh/Gemfile instead of Gemfile when it's under JH mode.

We need to do 2 things:

• Allow Gemfile to be set to a different path
• If we're under JH mode (can be determined by checking if jh/ directory exists, and EE_ONLY or FOSS_ONLY are both not defined. Namely: https://gitlab.com/gitlab-org/gitlab/-/blob/b3311e0d32310b889fa12bf768e8daebe0cb72fd/lib/gitlab.rb#L118-122)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.