WIP Fix gitaly-ruby bundle poisoning in CI

What does this MR do?

Fixes the way we launch gitaly in CI, to prevent gitaly-ruby from running with gitlab-ce's Gemfile.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The bundle environment of gitaly-ruby was being poisoned by the bundle environment of gitlab-ce. Sometimes, gitaly-ruby would still boot and function in spite of this poisoning. Sometimes it would fail to boot and this would then make lots of tests fail with mysterious "Deadline exceeded" errors.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitaly#1184 (closed)