OSS CI: use cache

[lf-]

It would be nice if dependencies were cached for OSS CI so that it would merely have to wait for glean itself to build. The goal here is CI down from ~20-30 minutes to 10. This should be doable with https://github.com/actions/cache and iirc haskell-ci does basically this.

We might also want to use e.g. a docker image uploaded to the glean repo's github registry to memoize the package installations as well.

[simonmar]

We did actually have this in the past, if anyone wants to go dig it up. The docker image was a bit of a pain to maintain which is why we moved away from it IIRC, but I'm not averse to going back if someone wants to try it.

[lf-]

Also once I get a little less busy I would not be especially averse to upstreaming our internal nix based build as another available CI target. It would not solve being able to build on antique ghc though, and needs some kind of cachix/garnix like object, so it's not really a complete solution.