main/grub: make grub-mount independent of grub
grub-mount is needed for os-prober, but adding grub-mount to os-prober's dependencies would require pulling grub, despite that grub-mount can work without grub itself.
grub-mount is needed for os-prober, but adding grub-mount to os-prober's dependencies would require pulling grub, despite that grub-mount can work without grub itself.