Summary:
Update to a newer python that builds on macOS Big Sur, make eden_scm depend on it and use it from PATH

python 3.8 requires libffi which is reference via its pkg-config setup,  however python's pkg-config libffi detection is broken (https://bugs.python.org/issue34823) with the documented workaround requiring an environment variable to be passed into its ./configure step, which is why this change also adds a feature to AutoconfBuilder

With the updated python in place I was able to remove disable_env_override_pkgconfig = 1
disable_env_override_path = 1 from the eden_scm config so that it actually uses the pkg-configs, PATHs and other settings from its dependencies. This should make future python and other dependency upgrades much simpler.

Reviewed By: HarveyHunt

Differential Revision: D32231261

fbshipit-source-id: a2b6addbe22f38e3d71618c802d2c6f836fdd86c