Without this change, GRUB installation on non-PC systems (such as
aarch64-linux) only works if boot.loader.grub.devices is set to exactly
`["nodev"]`. If boot.loader.grub.devices was any other value (including
the default `[]`), users got the error:
Died at /nix/store/an9ngv2vg95bdcy0ifsxlbkasprm4dcw-install-grub.pl line 586.
install-grub.pl verifies that if both $grub and $grubEfi are set, then
$grubTarget (e.g. i386-pc) and $grubTargetEfi (e.g. x86_64-efi) must
both be set, or the script will `die`. On non-PC systems, $grubTarget
is "".
When boot.loader.grub.devices is ["nodev"], $grub is set to null,
disabling non-EFI installation. But if a user has devices set for an
x86_64 config, or is using only mirroredBoots without setting devices,
they will hit this `die`.
This change sets $grub to "" if $grubTarget is "".
53135cc8c7