Mach is breaking when building GeckoView based on 81.0b2
https://bugzilla.mozilla.org/show_bug.cgi?id=1656993 makes it necessary
to run either ./mach create-mach-environment or set
MACH_USE_SYSTEM_PYTHON. The former is not easily for us to do as
missing Python packages need to get downloaded which breaks as we don't
allow that. So, we stick for now with the environment variable