Really fix use of NixOps with the merged Nixpkgs/NixOS tree

Note that there is a subtle difference in Nix that causes
nixos-rebuild to work and NixOps to fail:

  $ nix-instantiate '<nixos>' -A config.system.nixosVersion --eval-only
  "13.10pre34915.50f4822"

  $ nix-instantiate '<nixos/default.nix>' -A config.system.nixosVersion --eval-only
  error: opening file `/nix/var/nix/profiles/per-user/root/channels/nixos/.version': No such file or directory

Fixes NixOS/nixops#145.
Get rid of <nixpkgs>