turris-maintain: use tar command relative path for "maintain-config-backup" script
Reported on forum: https://forum.turris.cz/t/turris-os-6-0-is-released/17830/147
/bin/tar is not a symbolic link to busybox now, but to /usr/libexec/tar-gnu.
In your script /usr/bin/maintain-config-backup you use /usr/bin/tar as a command, but this link is now missing completely.