use nonlocal mem_tn variable

[jfennick]

Traceback (most recent call last):
  File "/home/jfennick/mambaforge-pypy3/envs/wic/lib/python3.10/site-packages/cwltool/job.py", line 518, in get_tree_mem_usage
    children = monitor.children()
  File "/home/jfennick/mambaforge-pypy3/envs/wic/lib/python3.10/site-packages/psutil/__init__.py", line 277, in wrapper
    raise NoSuchProcess(self.pid, self._name, msg=msg)
psutil.NoSuchProcess: process no longer exists (pid=31753)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/jfennick/mambaforge-pypy3/envs/wic/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
    self.run()
  File "/home/jfennick/mambaforge-pypy3/envs/wic/lib/python3.10/threading.py", line 1378, in run
    self.function(*self.args, **self.kwargs)
  File "/home/jfennick/mambaforge-pypy3/envs/wic/lib/python3.10/site-packages/cwltool/job.py", line 531, in get_tree_mem_usage
    if mem_tm is not None:
UnboundLocalError: local variable 'mem_tm' referenced before assignment

Using the --parallel flag, the mem_fn variables in the inner and outer scopes are not the same due to a missing global mem_tm line.

[codecov]

Codecov Report

Merging #1896 (adb7599) into main (9accbc5) will increase coverage by 1.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1896      +/-   ##
==========================================
+ Coverage   82.93%   83.95%   +1.01%     
==========================================
  Files          46       46              
  Lines        8163     8163              
  Branches     2168     2168              
==========================================
+ Hits         6770     6853      +83     
+ Misses        896      841      -55     
+ Partials      497      469      -28     

Files Changed	Coverage Δ
cwltool/job.py	81.48% <ø> (-0.20%)	⬇️

... and 8 files with indirect coverage changes

[jfennick]

Using nonlocal instead of global now.