Bug 1607470, unset __PYVENV_LAUNCHER__ for virtualenvs, r=firefox-build-system-reviewers,rstewart

The __PYVENV_LAUNCHER__ is set on macos to indicate which
python to use. Sadly, keeping this set confuses pip in
virtualenvs.
This corresponds to https://github.com/python/cpython/pull/9516.

Differential Revision: https://phabricator.services.mozilla.com/D59853

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/virtualenv.py

@@ -188,6 +188,9 @@ class VirtualenvManager(object):
         This should be the main API used from this class as it is the
         highest-level.
         """
+        # __PYVENV_LAUNCHER__ confuses pip about the python interpreter
+        # See https://bugzilla.mozilla.org/show_bug.cgi?id=1607470
+        os.environ.pop('__PYVENV_LAUNCHER__', None)
         if self.up_to_date(python):
             return self.virtualenv_root
         return self.build(python)