Assuming that the `MOZILLABUILD` environment variable is set, allow
invoking Mach from non-MozillaBuild terminals.

Note that MozillaBuild still needs to be installed, and the
`MOZILLABUILD` environment variable will have to be set.

For future reference: when I tried setting this up with Windows
Store's Python 3.9, I encountered issues when running binaries installed
via `pip`: it would fail with `abort: failed to load Python DLL
python3x.dll`.

Differential Revision: https://phabricator.services.mozilla.com/D133936