Run action on labeled, unlabeled

[sophie-warner-annalise-ai]

Hey, what do you reckon about allowing the action to be triggered when a label gets added? I have a label that I'd like to change my env vars.

Current flow

1. Open PR
2. Action runs
3. User manually adds label
4. Push another commit
5. Action runs with label based on env var

Proposed flow

1. Open PR
2. Action runs
3. User manually adds label
4. Action runs with label based on env var
5. Push another commit
6. Action runs with label based on env var