net-mgmt/librenms: start service in WWWDIR

The latest release uses relative paths for some includes
Our default path is /
That craps out the code like this:

Error Failed opening required 'includes/discovery/sensors/cisco-entity-sensor.inc.php'

re: https://github.com/librenms/librenms/issues/18794

PR: https://github.com/librenms/librenms/issues/18794