Correctly wati for output file with google benchamrk

bin/donkey_benchmark_execution_wrapper.py

@@ -16,5 +16,5 @@ if ret != 0:
 
 # If that was succesful, wait for the output file to be available on the filesystem
 # This step is necessary because the NFS synchronization is too slow for our workflow.
-while not os.path.isfile(sys.argv[2]):
+while not os.path.isfile(sys.argv[-1]):
     time.sleep(0.1)