auto-restart: fix interaction with pre-startcp tasks

[oliver-sanders]

Fix a small bug discovered whilst reviewing #7148.

Unlikely to occur, so not a valid blocker for that issue, but possible and easy to fix.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• no changelog needed - unreleased bug
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

Elegant!

[MetRonnie]

Seems like a good idea 👍