Installer can become confused when multiple `/boot/` partitions are present

[gibmat]

When attempting to re-install an IncusOS system, the boot-time PE binary check can become confused if two /boot/ partitions exist. (One from the previously installed IncusOS, and one from the install media.) This can result in an error about not being able to find the IncusOS EFI file.

I think the underlying issue is how systemd auto-mounts boot when it needs to pick from two partitions with the same label. We introduced some udev rules to handle this when the system is running, and might need to introduce a similar configuration for the initrd environment.