gpt-auto-generator: write fsck override unit to "middle" generator dir

gpt-auto-generator: write fsck override unit to "middle" generator dir

We generally let gpt-auto-generator write to the "late" generator dir,
so that the explicit fstab-generator can write to the "middle" one. And
I think we should leave it that way. But we also want to override the
generic systemd-fsck-root.service service potentially, and if we'd do
that in the "late" generator dir, it would be pointless, since that's
ordered *after* the static version hence would never be taken into
consdiration.

Hence clean this up: keep writing to the late dir for everything,
except for the fsck stuff.