Run acceptance tests directly against PRs in this repo

[alexwlchan]

Lifting a comment from #645 (comment):

Yeah we don't currently have an automated way of running acceptance tests against PRs aside from the manual workflow trigger in corp.

Something I've wanted for a bit but not had the time to explore further is adding a GitHub app / bot that listens for PRs here, downloads the repo at the branch, and runs the acceptance test flows (assuming that the author is a contributor or that we have explicitly approved running against a branch from a fork). This is an even further in the future thing, but would be a lot nicer UX for PRs here if / when we have it 🙂

This is the workflow in question: https://github.com/tailscale/corp/blob/main/.github/workflows/terraform-acceptance.yml

It runs automatically once a day, but it'd be nice to wire it up to run automatically.