bug#40999: GRUB prevents booting a degraded RAID1 array atop LUKS

[maxim . cournoyer]

On a system where:

1) Each disks comprising the array is fully LUKS encrypted
2) Each mapped disk is made part of a Btrfs RAID1 array

When attempting to boot the system after pulling out (in BIOS or using
the cable) the drive to simulate a complete disk failure, GRUB hangs,
prompting for the LUKS password of the disappeared drive and
(unsurprisingly) failing to open it.

This prevents booting in a degraded LUKS encrypted, Btrfs RAID1 on Guix
System.

Maxim