main/mariadb: fix plugin directory
When using MariaDB without services - in this case, in a Docker container
- mariadb-install-db will fail, as it tries to load /usr/lib*/plugin and /usr/lib*/mysql/plugin. None of which exist, in a MariaDB installation.
The correct path would be /usr/lib*/mariadb/plugin.
Failure to load plugins, leads to inability to load PAM and inherently fails the installation.