Use popen to have a reference on ML worker process

app.py

@@ -21,13 +21,15 @@ ml_worker = None
 def run_ml_worker(url, api_key, hf_token):
     global ml_worker, previous_url
     previous_url = url
-    ml_worker = subprocess.run(
+    ml_worker = subprocess.Popen(
         [
             "giskard", "worker", "start",
             "-u", f"{url}", "-k", f"{api_key}", "-t", f"{hf_token}"
         ],
         stdout=open(LOG_FILE, "w"), stderr=subprocess.STDOUT
     )
+    args = ml_worker.args
+    print(f"Process {args} exited with {ml_worker.wait()}")
 
 
 def stop_ml_worker():