We actually do not depend on any input here. Only if a
nixosConfiguration should be instantiated, but it is already
expected that the user will provide a (more up-to-date) nixpkgs
flake that we use.
As the home-manager depends on the actual used nixpkgs version
and is not always used, remove it from the base dependencies but
use still use it implictely if a home-manager input exists.
