Tweak pre-commit autoupdate CI workflow

[altheaden]

This PR makes a few tweaks to the new pre-commit CI workflow. This workflow now skips the push and PR-creating steps when no changes are present (i.e., when pre-commit is already up-to-date). Also, PRs created via this action will automatically be assigned reviewers and a ci label.

Checklist

• Testing comment in the PR documents testing used to verify the changes

[altheaden]

Testing

I tested this pretty extensively on my own fork of mache. I had some issues understanding the syntax related to the GITHUB_ENV variables. I ended up testing two scenarios:

1. There are no updates to pre-commit when the workflow runs. This causes the push and pr-create steps to be skipped as expected.
2. There are updates to pre-commit when the workflow runs. This causes all steps to run, creates a PR, and adds labels and reviewers as expected.
   @xylar let me know if you can think of any other scenarios that might be useful to test.

[xylar]

@altheaden, oh, you are so clever! This looks great!

[xylar]

Your testing sounds fine. I can't think of anything else to try out.