Add EnvironmentFile=/etc/sysconfig/mariadb to MariaDB service's unit file

It's normal and expected for systemd services to provide a way to set environment variables without direct modification of the servic's until file (that may be lost during upgrade etc). In case of gardb it's done via /etc/sysconfig/garb, in case of MySQL server it's /etc/sysconfig/mysql, see https://dev.mysql.com/doc/refman/8.0/en/using-systemd.htm.

For some reason MariaDB service's unit file included with RPMs for CentOS 7 etc does NOT contain any predefined EnvironmentFile setting. I think it should be added, so service reads whatever definitions (like LD_PRELOAD) are present in /etc/sysconfig/mysql (for compatibility) and/or /etc/sysconfig/mariadb without the need to change unit file or do extra "systemctl set-environment ..." calls,