nixos/udev: Fix up readlink and basename as well

In 8ecd3a5e

, we fixed up the FHS paths for stage 1, but unfortunately we
have a similar udev rules generator twice one for the initrd and one
without. So we might need to refactor this in the future.

For now, let's just fix the references to readlink and basename in the
udev module as well until we have properly addressed this.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Fixes: #12722