eval_started event: change interface to traceID\tjobsetID

I was not going to break the interface until I noticed
the current implementation uses the string literal \t.

src/script/hydra-notify

@@ -98,6 +98,7 @@ $listener->subscribe("build_queued");
 $listener->subscribe("build_started");
 $listener->subscribe("cached_build_finished");
 $listener->subscribe("cached_build_queued");
+$listener->subscribe("eval_started");
 $listener->subscribe("hydra_notify_dump_metrics");
 $listener->subscribe("step_finished");