Okay, I found it myself:
By adding a -D parameter to the procfile, I can determine the environment play is running in.
So my procfile's going to look like:
web: play run --http.port=$PORT $PLAY_OPTS
worker: play run --http.port=$PORT $PLAY_OPTS -Dprocesstype=worker
By using
System.getProperty("processtype");
I can ensure, that i am on a worker dyno and process my jobs only then.