mariadb: disable auth_pam plugin on darwin

Fixes #69034

This plugin doesn’t work right for us now, needs to be disabled. It
was added first in 10.3.18:

https://github.com/MariaDB/server/commit/91fdb931fa2f734d54acadeb3e9c860a26543f04#diff-7cea40646c6b8df9a67a3eac4eec9bc6