[PR Label enforcer] When a label is added to a PR, trigger the worflow to re-verify the existence of the label

[ZainRizvi]

Currently folks have two options for getting their "missing label" ci check to pass:

1. Make a new commit
2. Manually rerun the workflow (if they even have permission to do so)

Instead, the CI should do this automatically.

This can be done in one of two ways:

1. Quick fix: Add a on:pull_request.labeled trigger to the lint.yml workflow
2. Proper fix: Move the label check job to it's own workflow, trigger it on pulls and on:pull_request.labeled, and ensure it can block CI