[CI] Environments cannot be cleaned if MR is merged prior to automatic cleanup

Summary

Example: in gitlab-org/charts/gitlab!2342 (merged)

1. Environments were active/running
2. Automatic cleanup after ~1h had not yet triggered
3. MR was merged
4. Cleanup jobs were triggered automatically upon merge, but cleanup failed: fatal: couldn't find remote ref refs/heads/bump-charts-v5.7.0

Note: I manually deleted these namespaces in the meantime.

Acceptance criteria

• CI environments properly clean up when MR is merged prior to automatic clean up