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.

[michaelst0]

We've run into this issue a couple of times error: Failed to create /run/ostree: File exists.

We're running on RPi 4 compute module with Fedora IoT 41 using SD card for storage. We think that the cause is powering off during the boot stage. The next boot can end up in the failed state.

Here's a screenshot my colleague took: