mxgrub: Add switch no-nvram to grub-install for UEFI

In case a drive with an EFI System Partition (ESP) is put into a system
that boots in legacy mode, running `grub-install --target=x86_64-efi`
will fail, as `efibootmgr` fails to update the NVRAM. Therefore, pass
`--no-nvram` to `grub-install`, which skip running efibootmgr.

    --no-nvram             don't update the `boot-device'/`Boot*' NVRAM
                           variables. This option is only available on EFI
                           and IEEE1275 targets.

As a downside, the administrator needs to go int the firmware menu, and add the
entry in the boot manager manually.

mxgrub/mxgrub

@@ -321,7 +321,7 @@ sub cmd_install {
 			print "GRUB for UEFI is already installed.\n";
 		}
 		else {
-			sys 'grub-install','--target=x86_64-efi',$root_disk and exit 1;
+			sys 'grub-install','--target=x86_64-efi','--no-nvram',$root_disk and exit 1;
 		}
 		sys 'umount','/boot/efi' and exit 1;
 	}