Backport of 'Stop jobs without a fetch from borrowing the git object pool' and 'Fix downstream pipelines git alternates'

What does this MR do and why?

This MR backports two CI fixes to 19-5-stable-ee. The branch has the git object pool change from !259066 (merged), but not these fixes. Because of that, spec/keeps/overdue_finalize_background_migration_spec.rb fails on 19-5 stable and backport pipelines with unable to normalize alternate object path.

• !261679 (merged): jobs that take the repository from artifacts no longer borrow from the pool.
• !260583 (merged): trigger jobs no longer forward the pool path to downstream pipelines. The job_arguments spec refactor from that MR is not included.

Recent failures:

• https://gitlab.com/gitlab-org/gitlab/-/jobs/17263886799
• https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/17263908497

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.

Related to https://gitlab.com/gitlab-org/quality/test-failure-issues/-/work_items/45437