galera: fix permission of temporary log file for mariadb 10.1.21+

Since MariaDB/server@8fcdd6b0ecbb966f4479856efe93a963a7a422f7,
mysqld_safe relies on a helper subprocess to write into log files.
This new logging mechanism expects log file to be writable by the
user configured to run mysqld.

Fix the generation of temporary log file accordingly.