truncate disk name for grub-install

i.e. /dev/nvme0n1p1 -> /dev/nvme0n1

post-debootstrap-installer.sh

@@ -43,7 +43,7 @@ else
     apt-get -y install grub-pc
 
     boot_disk=$(mount | grep ' on /boot ' | cut -d' ' -f1)
-    grub-install "$boot_disk"
+    grub-install "${boot_disk::-2}"
 fi
 
 # edit uuids in fstab