Using environment variable to specify backend is too restrictive (ex: use inside application server or junit)

Using -Dflogger.backend_factory=com.google.common.flogger.backend.slf4j.Slf4jBackendFactory#getInstance (for exemaple) to specify the backend to use is not usable for application running on same JVM with others like it is for tomcat or any application server. Even if it might be possible if one has control of the server, it will impact all application and could be problematic.
Could be imagine using a config file using a pre-defined name that would contains configuration that would be checked before looking at environment variable ?