Only run actions dependency-review-action if running in a PR action.

This broke when run as part of action for commit on main and on a
preview branch:
- https://github.com/zed-industries/zed/actions/runs/12793068921/job/35664998296
- https://github.com/zed-industries/zed/actions/runs/12793045639

Originally introduced in:
- https://github.com/zed-industries/zed/pull/21424

But was only tested with `merge_group` which has since been reverted.