race condition in allowlist / action updates

[raboof]

what should have happened:

• dependabot commit 1cfd733 updated .github/actions/for-dependabot-triggered-reviews/action.yml
• .github/workflows/update_actions.yml updates actions.yml and approved_patterns.yml

but what actually happened:

• dependabot commit 1cfd733 updated .github/actions/for-dependabot-triggered-reviews/action.yml
• manual commit ba7238d updated actions.yml
• .github/workflows/update_composite_action.yml commit d112413 updated approved_patterns.yml and overwrote(!) .github/actions/for-dependabot-triggered-reviews/action.yml

so a race condition. we should think about how we can avoid this - possibly we can combine the two actions into one, making sure we always first add information from .github/actions/for-dependabot-triggered-reviews/action.yml to actions.yaml and only after that do the update in the opposite direction.

Originally posted by @raboof in #857 (comment)