Add first class support for Cachix

[blaggacao]

i.e. branch into a ./run/cache_cachix.sh.

The cachix CLI makes use of the Cachix API for (still) faster interaction with the cache.

[nrdxp]

If there is some way to upload to cachix with a standard nix copy I'd prefer that, rather than having a special code path.

[blaggacao]

https://blog.cachix.org/posts/2022-12-19-zstd-compression/