check_file_latency doesn't work with filepaths

The check_file_latency plugin raises a TypeError when attempting to resolve -p/--path point to a file:

$ python3 -m igwn_monitor.plugins.check_file_latency -p /tmp/x509up_u1000
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/plugins/check_file_latency.py", line 213, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/plugins/check_file_latency.py", line 194, in main
    status, message = check_file_latency(
                      ^^^^^^^^^^^^^^^^^^^
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/plugins/check_file_latency.py", line 113, in check_file_latency
    path = find_latest(Path(path), key=file_end)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/plugins/check_file_latency.py", line 68, in find_latest
    return sorted(glob.iglob(path), key=key)[-1]
                  ^^^^^^^^^^^^^^^^
  File "/home/duncan/opt/mambaforge/envs/py311/lib/python3.11/glob.py", line 48, in iglob
    root_dir = pathname[:0]
               ~~~~~~~~^^^^
TypeError: 'PosixPath' object is not subscriptable