nixos/systemd: Explicitly put default path packages after others

This fixes the dhcpcd issue in https://github.com/NixOS/nixpkgs/issues/76969,
which was exposed by https://github.com/NixOS/nixpkgs/pull/75031
introducing changes in the module ordering and therefore option ordering
too.

The dhcpcd issue would also be fixable by explicitly putting
dhcpcd's paths before others, however it makes more sense for systemd's
default paths to be after all others by default, since they should only
be a fallback, which is how binary finding will work if they come after.