Most CI jobs are not fully utilizing magic-nix-cache due to Github Actions Cache API rate limits

[yfyf]

I wanted to check why ./build vm always takes so long on the CI pipeline and realized it's not a matter of cache misses, but simply due to the fact that it needs to build almost ~2000 nix derivations and hits the Github API rate limits at around the ~800th cache download, at which point the cache is disabled.

Example:
https://github.com/dividat/playos/actions/runs/12046335677/job/33586605917?pr=207

It seems that PlayOS has "outgrown" the magic-nix-cache. It might make sense to consider either Cachix or self-hosting using attic or some other approach for speeding up the builds.

cc @knuton