Maintenance job should not be launched if the repo already has a runn…

[qiuming-best]

…ing one

Thank you for contributing to Velero!

Please add a summary of your change

Does your change fix a particular issue?

Fixes #(issue) https://github.com/vmware-tanzu/velero/issues/7751

Please indicate you've done the following:

• [ ] Accepted the DCO. Commits without the DCO will delay acceptance.
• [ ] Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• [ ] Updated the corresponding documentation in site/content/docs/main.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 42.85714% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 58.65%. Comparing base (a798182) to head (e91d9b9). Report is 5 commits behind head on main.

Files	Patch %	Lines
pkg/repository/manager.go	0.00%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7752 +/- ## ========================================== - Coverage 58.66% 58.65% -0.01% ========================================== Files 344 344 Lines 28723 28730 +7 ========================================== + Hits 16849 16853 +4 - Misses 10445 10448 +3 Partials 1429 1429 ```

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