Restructure to be runnable as a GitHub action

[delroth]

There are several drawbacks to the current "crawling" approach used by this tool:

• Easily runs into quota issues
• Can't process more than the last 1K PRs by ID (due to GH search API limitations, maybe?)
• High latency since the whole set of open PRs needs to be recrawled on every run

I suggest the following complementary mode of operation:

• Add a mode to run only on one PR, whose ID is provided on the command line.
• Add all the scaffolding to be able to run this tool as a GitHub action.
• Register this action to run on PR review events on nixpkgs.

Happy to take care of this myself (want to share commit privileges / future maintenance? :p), I really just want this to finally happen automatically on nixpkgs.

[Artturin]

I added you as as collaborator with maintainer level permissions

[delroth]

Thanks! Do you still want to review those changes or are you giving me the green light to commit straight to master / self-merge?

[Artturin]

You can self merge

A few things to keep in mind

The program should stay runnable without the action
And it should eventually be runnable on non-nixpkgs repos

[delroth]

Pushed everything needed so this can be used as nix-community/label-approved@v1 in GH actions. NixOS/nixpkgs#264633 is the nixpkgs side of things to start using this action, then I can stop running the bot :)