Add binary cache

[brckd]

I noticed that especially testbeds take a long time to build. This could be avoided by caching build results. There was a Magic Cache before, but it was removed in #745 because it was deprecated. Feel free to suggest and discuss various solutions.

[brckd]

I recently tried out a private Cachix cache in my own configuration and can recommend it. I'm not aware of any rate limits and can also be used in local builds. They also seem to offer the most generous free tier.

The following steps would need to be added to jobs in order to be cached.

      - name: Install Nix
        uses: cachix/install-nix-action@v25
        with:
          nix_path: nixpkgs=channel:nixos-unstable
      - name: Set up Cachix
        uses: cachix/cachix-action@v14
        with:
          name: stylix
          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
      - name: Builds to be cached
        run: nix build .\#whatever

The stylix.cachix.org cache could then be added to the flake to use it for both automated and local builds. The same entry could be added to other configurations that use Stylix.

#flake.nix
{
  nixConfig = {
    substituters = [ "stylix.cachix.org" ];
    trusted-public-keys = [ "..." ];
  };
}

[trueNAHO]

Related: #749

[danth]

There's also the option of self hosting something like Attic if the free version of Cachix too limiting. I already have a server I'm using for a few personal things which would be able to run this.

On the other hand, it's probably better to use Cachix if possible as all the infrastructure is managed for us + it's something a lot of people are already familiar with.

It might be useful to get an estimate of the size of all the testbeds and other packages before deciding which service to use - we need something which can hold at least a few commits of history before garbage collecting.