Fix plugin directory in the config file

Attempt to install the plugin currently fails with:

$ echo "INSTALL PLUGIN sql_errlog SONAME 'sql_errlog';" | mysql -uroot
ERROR 1126 (HY000) at line 1: Can't open shared library '/usr/lib64/mysql/plugin/sql_errlog.so' (errno: 11, cannot open shared object file: No such file or directory)

It's not clear to me whether this file with path spec is still needed at all, especially in versions that do not have a build for SCL (if it was added for that reason at all). Maybe this base.cnf might be removed, but I didn't do proper analysis to be able to say that it's possible. So, applying minimal patch meanwhile.

root-common/usr/share/container-scripts/mysql/pre-init/my-base.cnf.template

@@ -1,5 +1,5 @@
 [mysqld]
 datadir = ${MYSQL_DATADIR}
 basedir = ${MYSQL_PREFIX}
-plugin-dir = ${MYSQL_PREFIX}/lib64/mysql/plugin
+plugin-dir = ${MYSQL_PREFIX}/lib64/mariadb/plugin