backuptool: Execute check_blacklist from the current directory

 * For some odd reasons executing `cd /system/addon.d` makes the system
   hang and unmount error:
     umount: /system_root: Device or resource busy
 * Don't change directory to not allow the system partition hang

Change-Id: I3d30bdc59c2f05d16823e99046c1dce2e1e6eb73