Fix rauc hook to restore machine-id in new grub environment

[claplace]

Without a machine-id, the next boot will be considered as a first boot, and any external data disk will be disabled.

Fixes #3247.

After installing my raucb update bundle and a reboot:

Jun 29 16:34:38 homeassistant kernel: Kernel command line: BOOT_IMAGE=(hd0,gpt4)/Image root=PARTUUID=a3ec664e-32ce-4665-95ea-7ae90ce9aa20 rootwait zram.enabled=1 zram.num_devices=3 systemd.machine_id=6c631b4986cb426f9b7860df4ce10629 fsck.repair=yes console=ttyS0 console=tty0 rauc.slot=B

The machine-id has been conserved 👍