home-manager-auto-upgrade: respect nix.package

The generated auto-upgrade script took its nix from `pkgs.nix` regardless
of `nix.package`, so a configuration that sets an alternative Nix ran the
scheduled upgrade with a different implementation than everything else the
module system generates, and pulled a second nix into the closure.

Resolve it the way modules/services/nix-gc.nix already does, falling back
to pkgs.nix only when nix.package is null.