use nonlocal mem_tn variable

Author: jfennick

cwltool/job.py

@@ -513,6 +513,7 @@ def process_monitor(self, sproc: "subprocess.Popen[str]") -> None:
         mem_tm: "Optional[Timer]" = None
 
         def get_tree_mem_usage(memory_usage: MutableSequence[Optional[int]]) -> None:
+            nonlocal mem_tm
             try:
                 with monitor.oneshot():
                     children = monitor.children()