fix: check correct dependency package grub-common instead of grub-pc
grub-mkimage is nowadays provided by grub-common. To verify:
dpkg -S `which grub-mkimage`
grub-common: /usr/bin/grub-mkimage
There's no need to require grub-pc being installed. That package can not be installed in my custom image that already comes with grub-efi-amd64 for dual BIOS + EFI booting compatibility.
grub-pc Conflicts: with grub-efi-amd64.