Think about explicit GitHub Actions cache support

[zombiezen]

Following on from #43, it would be good to support GitHub Actions cache in some way. It would certainly make 256lights/zb-stdlib#4 easier. I'll need to dig deeper into the cache API to understand what the correct integration point is, but as long as I can get key-value storage, it should be straightforward.

[zombiezen]

Looks like someone wrote some unofficial docs: https://github.com/tonistiigi/go-actions-cache/blob/1a5174abd0559b23ff2b1c2ded57c5f6a0d8669c/api.md