By preserving the `MAVEN_DEP_DESTINATION=/kafka/libs` and not overriding it with the value of `KAFKA_CONNECT_PLUGINS_DIR` (`/kafka/connect`), it will be easier for clients to  reuse the `docker-maven-download.sh` script when building descendant images.

The proposed solution is to use the `KAFKA_CONNECT_PLUGINS_DIR` env var in the download script.