This task is similar to CONC-449, but this task is for the server itself, and not for MariaDB Connector/C.
Currently, if the $MYSQL_HOME environment variable is set, then MariaDB reads the $MYSQL_HOME/my.cnf configuration file, if it exists:
https://github.com/MariaDB/server/blob/mariadb-10.4.11/mysys/my_default.c#L1206
https://mariadb.com/kb/en/library/configuring-mariadb-with-option-files/#default-option-file-locations-on-linux-unix-mac
To further MariaDB's branding, I think MariaDB should similarly check $MARIADB_HOME/my.cnf, if the $MARIADB_HOME environment variable is set.
UPDATE:
The approach should be to only use MARIADB_HOME if set. MYSQL_HOME should only be used if MARIADB_HOME is not set. See also the similar change CONC-537