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
FixesNixOS/nixops#145.