pw_presubmit: Accept either --clear or --clean
pw_presubmit: Accept either --clear or --clean

Have either --clear or --clean clear the environment and exit.

Change-Id: I112d7f893bbde34d87ef675b512b3c02ece1bb69

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed