Add gitaly-backup to gitlab-task-runner for backups

What does this MR do?

In gitlab-org/gitlab!64834 (merged) a new setting was added to locate gitaly-backup. To make this easier for CNG the default for this setting is a search of $PATH. So here we just need to make sure the binary is on system path. The binary receives all configuration by backup.rake.

gitaly-backup is a new binary to perform repository backups for gitaly/praefect. It's currently behind a feature flag gitaly_backup (gitlab-org/gitlab#333034 (closed)).

Related issues

gitlab-org/gitlab#35862 (closed)

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
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated

Testing

• Turn on feature: (rails console)

  Feature.enable(:gitaly_backup)

• Take a backup: (shell)

  gitlab-backup create

If repository backups commence this is enough to evaluate that the setting is working as expected.