prepare-root: Avoid retries

[cgwalters]

I've seen some failure cases where when ostree-prepare-root.service fails, it ends up getting retried (not totally sure how)...and what happens is we get:

error: Failed to create /run/ostree: File exists

And that combined with other noise makes the real error scroll off the TTY...

So...I think we need to dig in and try to;

• Ensure our unit doesn't get rerun
• Try to add cleanup/idempotence on failure so we can rerun (a good bit harder)

[jlebon]

#3220 should definitely help. Hard to tell if it fixes it completely unless you can get a reproducer for the specific issue you've hit.