The cachix directory simple captures the output of sudo cachix use for the
developers personal cache, as well as the nix-community cache. You can easily
add your own cache, assuming the template lives in /etc/nixos, by simply
running sudo cachix use yourcache.
These caches are only added to the system after a nixos-rebuild switch, so it
is recommended to call cachix use divnix before the initial deployment, as it
will save a lot of build time.
In the future, users will be able to skip this step once the ability to define the nix.conf within the flake is fully fleshed out upstream.