Fix gitaly-ruby bundle poisoning in CI (10-8-stable)

What does this MR do?

This ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18853 into 10-8-stable. Changes how Gitaly is spawned during CI, to fix test failures.

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• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
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• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

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