build zip of .mpy files with GitHub Action, resolve #270

[rrotter]

This is a fairly trivial GH Action that automatically runs make compile and uploads the zip (for an example of what this looks like, see Artifacts here). As configured this will only run on merge/push to the master branch. Only supports CircuitPython 8, would be easy to add other versions if need be.

This was requested in #270.

[xs5871]

Couple of questions:

• We would need to mention in documentation where to find and download that for it to be usefull to anyone, right?
• Building on every push seems a bit wastefull, but I guess we could design some kind of check to only build and save if the source subtree got updated?
• I'm not familiar enough with github actions. Is it common practice to pull dependencies (i.e. download mpy-cross) for every build? Or is there a persistant container / venv solution?

[rrotter]

• Yes, if this is merged we should add some help text explaining what these builds are and how to find/use them. The artifacts are publicly discoverable (once there were several builds it would look much like the ZMK build workflow). I'll be happy to add something to the docs if that's the only blocker. Though, that might be better placed in a second PR. There would be some question where to put it and how to stage it (i.e. could be anything from making mpy files the new recommended install for new users, to a footnote on a page about nrf52 and it's small flash, etc)
• That would be tricky. The action only has the current commit checked out, no view into the history. Also one would need to check if /kmk changed since the last build, not the prev commit (since a push could contain any number of commits). I'm not sure if you're concerned about wasting CPU or storage space but:
  • To be clear this isn't on every push it's set to run on every push to the master branch.
  • This uses 6-10 seconds of CPU time (compared to the Test action that takes about 90 seconds to run, and isn't restricted to one branch).
  • The zip is under 100K, and GitHub deletes them after 90 days (their policy, not my setting), and AFAICT storage is either limited to 500MB or unlimited because it's a public repo (the docs are a little confusing on this point). By my back-of-the-envelope calculations it would take over 50 builds every day to exhaust the 500MB quota before the 90 day timeout deletes the builds anyway.
• Good point. I updated the workflow to cache mpy-cross.

Latest force push cleans up the naming of the Action, the naming of the zip (uses short sha and not the full sha), and caches mpy-cross as mentioned above.

[leiserfg]

• That would be tricky. The action only has the current commit checked out, no view into the history.

It's not so complicated, just use https://github.com/tj-actions/changed-files