Do not set `HOME` in `scripts/gitaly-test-spawn`

What does this MR do?

This changes a very old behavior that seems to be wrong:

• overwrite of HOME: https://gitlab.com/gitlab-org/gitlab/-/blob/94ff08ba3d3aa7a1274e3fc919fcb6bf789e09fd/scripts/gitaly-test-spawn

This got properly resolved by Jacob with d291f69f, and Remy with setting bundle path:

• This breaks an ability to re-use a global config, and breaks the Gitaly ability to properly bundle install all dependencies and re-use them in a system.
• This makes the gitaly for tests also slower to execute if gems sharing is desired.

Like with:

bundle config set --global path ~/.gems/ruby/...

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because internal change.
• Code review guidelines