Update our test backup to be a recent one from master

What does this MR do?

This avoids restoring old backups into the 13.5 codebase, which doesn't work (see the change: in gitlab-org/gitlab!42967 (diffs, comment 419794489))

Also updated the way we provide the timestamp prefix so we only have one location to update in the code going forward, AND it can be overwritten via ci variable if need be.

Related issues

Fixes the errors seen in the master failed pipeline: https://gitlab.com/gitlab-org/charts/gitlab/-/jobs/759826652

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened