nixos/lib/make-disk-image: introduce `noNixStore` option
Sometimes, we want to use the host nix store for everything because it's
more convenient.
To do so, we need to "not install" the nix store in the target disk
image.
We trick nixos-install by mount binding the host nix store into the
$root/nix folder and letting Nix do the idempotency thing.
