Revert "simplify boot disk lookup even more"

This reverts commit 65e7d0eb92.

post-debootstrap-installer.sh

@@ -38,12 +38,12 @@ cat /hardware-setup/authorized_keys/* > /etc/dropbear/initramfs/authorized_keys
 # install grub
 if [ -d /sys/firmware/efi ]; then
     apt-get -y install grub-efi-amd64
-
     grub-install
 else
     apt-get -y install grub-pc
 
-    grub-install "$(findmnt --noheadings --output SOURCE /boot)"
+    boot_disk="/dev/$(lsblk --noheadings --output PKNAME $(findmnt --noheadings --output SOURCE /boot))"
+    grub-install "$boot_disk"
 fi
 
 # edit uuids in fstab