Fix unmount in cleanup.sh

cleanup.sh

@@ -7,5 +7,5 @@ sudo umount ${FS}/dev/pts || true
 sudo umount ${FS}/sys/kernel/security || true
 sudo umount ${FS}/sys || true
 sudo umount ${FS}/proc || true
-sudo umount mnt || true
+sudo umount ${MNT} || true
 sudo rm --one-file-system -rf ${FS} ${ISO} ${MNT} ${DISK}