[ComposeFS backend] Series of rebases/updates into bootc rollback breaks bootc (persisting through reboots)

Steps to reproduce: - Have a bootc system with the composefs backend (I used [my project](https://github.com/Smujb/amethyris) when I discovered this but I also tested it with [apollo](https://github.com/apollo-linux/apollo)) - Rebase to a new image (e.g. `ghcr.io/smujb/amethyris`) - Before rebooting, rebase again to a _different tag_ which points to the same image (e.g. `ghcr.io/smujb/amethyris:latest`). This will correctly exit as the target image is the same. - Run `bootc rollback` - Run `bootc status` - Reboot your computer - Run any of: `bootc status`, `bootc upgrade`, `bootc rollback`, `bootc rebase`. - Error along the lines of: `error: Rollback: Rolling back composefs: Getting composefs deployment status: Getting composefs deployment status: Multiple extra entries in /boot, could not determine rollback entry` Additionally, `find /boot` will have duplicate entries now (which are not present after a successful update) Depending on the exact configuration of the system beforehand, `bootc rollback` and `bootc status` may error out. But consistently, regardless of whether they succeeded, bootc will break entirely after rebooting.