WIP: Resolve "Gitaly bundle isolation not working in CI"

What does this MR do?

Before (https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/50352403):

• setup-test-env takes 3 minutes to run
• artifacts.zip is 63MB
• unzipped tmp/tests size: 187MB

Uploading artifacts...
tmp/tests: found 2730 matching files               
config/secrets.yml: found 1 matching files         
Uploading artifacts to coordinator... ok            id=50352403 responseStatus=201 Created token=2JriUpLp

After (https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/50384571):

• setup-test-env takes 7 minutes to run
• artifacts.zip is 151MB
• unzipped tmp/tests size: 371MB

Uploading artifacts...
tmp/tests: found 13495 matching files              
config/secrets.yml: found 1 matching files         
Uploading artifacts to coordinator... ok            id=50384571 responseStatus=201 Created token=B-djuU3C

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

This changes makes the artifacts from setup-test-env 2.4x heavier since gems are now installed under tmp/tests/gitaly/ruby/vendor.

Why was this MR needed?

To make sure gitaly-ruby's gems bundle is isolated from GitLab's one.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #42624 (closed)