pw_env_setup: Clear __PYENV_LAUNCHER__ pre- GN run

On some MacOS Python distributions, the presence of a __PYENV_LAUNCHER__
env variable can cause pip installs to go into the wrong Python
environment (in addition to messing up venv creation).

Change-Id: Ifd3a45a0294910c6288451ce4574f4344f942124
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/42360
Reviewed-by: Rob Mohr <mohrr@google.com>
Pigweed-Auto-Submit: Joe Ethier <jethier@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_env_setup/py/pw_env_setup/virtualenv_setup/install.py b/pw_env_setup/py/pw_env_setup/virtualenv_setup/install.py
index 8a2b699..5f576aa 100644
--- a/pw_env_setup/py/pw_env_setup/virtualenv_setup/install.py
+++ b/pw_env_setup/py/pw_env_setup/virtualenv_setup/install.py
@@ -254,6 +254,7 @@
             env.set('VIRTUAL_ENV', venv_path)
             env.prepend('PATH', venv_bin)
             env.clear('PYTHONHOME')
+            env.clear('__PYVENV_LAUNCHER__')
             with env():
                 for gn_target in gn_targets:
                     install_packages(gn_target)