We no longer support them both together, so this is now safe to do. By
checking --local first, we avoid error out about an invalid --with-hg
script if --local was also given (we instead tell the user that the
options are mutually exclusive).
I also had to wrap the 'binpath' we pass to setattr() in _strpath() to
keep python3 run-tests.py -l working. That change also made `python3
run-tests.py -l --chg` work, which was the reason for this series.