nixos/nix-daemon: optionally (on by default) check nix.conf can be read

* checks using package providing the nix-daemon that we'll be using
* made optional (unlike some other config checks) "just in case":
  since this requires running the new Nix on the builder, this
  won't work in a few (AFAIK very uncommon) situations such as
  cross-compiling NixOS or using `include` directives in nix.conf

This does rely on Nix2 but not by the builder.
Since we only offer Nix2+ in-tree this should be fine,
and may otherwise be required anyway.