Bug 1877347 - Replace `distutils.util.strtobool` with a custom `strtobool` within `mach` r=firefox-build-system-reviewers,glandium

The migration advice (https://peps.python.org/pep-0632/#migration-advice)
suggests reimplementing the functionality as per the docs (https://docs.python.org/3.9/distutils/apiref.html#distutils.util.strtobool)
which is exactly what I've done here.

Differential Revision: https://phabricator.services.mozilla.com/D199950