bootctl: add A/B fallback for sd-boot updates (#41650)

On `bootctl install`, two EFI boot entries are registered: one for the
primary sd-boot binary and one for a fallback. On `bootctl update`, the
existing primary binary is rotated to the fallback path before the new
version is installed, so the fallback entry always points to the
previous known-good binary.

```
$ sudo bootctl install
...
Created EFI boot entry "Linux Boot Manager".
Created EFI boot entry "Fallback Linux Boot Manager".
$ sudo bootctl update
Copied "/boot/EFI/systemd/systemd-bootaa64.efi" to "/boot/EFI/systemd/systemd-boot-fallbackaa64.efi".
Copied "/usr/lib/systemd/boot/efi/systemd-bootaa64.efi" to "/boot/EFI/systemd/systemd-bootaa64.efi".
$ efibootmgr
...
Boot0004* Linux Boot Manager        HD(...)/\EFI\systemd\systemd-bootaa64.efi
Boot0005* Fallback Linux Boot Manager     HD(...)/\EFI\systemd\systemd-boot-fallbackaa64.efi
```

Fixes: #23805