nixos-install: Fix password prompt failing outside NixOS.

When bootstrapping from other distributions, nixos-install is unable to find
various tools in the chroot since their paths aren't aware of NixOS conventions.

This makes a small change to existing code by specifying nixpkgs/nixos instead
of just nixos when running nix-instantiate in the chroot. I haven't tested this
outside of bootstrapping, but the same specification is used elsewhere in the
code so I don't see why it wouldn't work.