use more modern and reliable tools for boot disk

post-debootstrap-installer.sh

@@ -42,8 +42,8 @@ if [ -d /sys/firmware/efi ]; then
 else
     apt-get -y install grub-pc
 
-    boot_disk=$(mount | grep ' on /boot ' | cut -d' ' -f1)
-    grub-install "${boot_disk::-2}"
+    boot_disk="/dev/$(lsblk --noheadings --output PKNAME $(findmnt --noheadings --output SOURCE /boot))"
+    grub-install "$boot_disk"
 fi
 
 # edit uuids in fstab