Current versions of xtrabackup-v2 and mariabackup support the option
--innodb-data-home-dir, but this parameter is not passed to them from
the SST script, since the SST script does not receive this information
from mysqld. The transfer of this information to the SST is already
fixed by the MDEV-10754 patch, but we need to process it in the SST
script. Also, we should take into account that on the joiner side
the corresponding information is not read yet from the configuration
file (at the mysqld side) during the start of SST, so the script must
itself read it.

https://jira.mariadb.org/browse/MDEV-10756