pyoxidizer: add the user site to sys.path on macOS
This allows 3rd party extensions that are installed with pip to be picked up,
similar to what we do on Windows. PyOxidizer has a bug that prevents this from
working without this extra help (see 95af358fcdfe), though it appears there's
another wrinkle here with sys._framework too.
I needed this to see if the problem loading the keyring extension on Windows
also occurs on macOS (it doesn't).
Differential Revision: https://phab.mercurial-scm.org/D11452