sympl-backup triggers `tar` warnings

https://forum.sympl.host/t/backups-tar-warning-about-non-optional-arguments/44

Problem Description

When doing backups, the following message is shown, with the backup succeeding:

Creating archive using 'DRIVER_TAR_GZ'...
  tar: The following options were used after any non-optional arguments in archive create or update mode.  These options are positional and affect only arguments that follow them.  Please, rearrange them properly.
  tar: --no-recursion has no effect
  tar: Exiting with failure status due to previous errors
Checking TOC of archive file (< real file, > archive entry)...

This is due to changes to tar in Buster.