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[1] loading the keyring extension on Windows also occurs on macOS (it doesn't). [1] https://github.com/indygreg/PyOxidizer/issues/445 Differential Revision: https://phab.mercurial-scm.org/D11452