This fixes the following eval error:
```
error: nodePackages.node-red cannot be found in pkgs
```
when having `services.node-red.enable = true;` without specifying
`services.node-red.package`, just like the nixos VM test.
Follow-up of f509382c11, which attempted
to fix this as well.
Breakage introduced in 0a37316d6c.
22bbd1834e