Setting PIP options can break environment setup

[henryiii]

Description

Setting a pip option, like PIP_USE_DEPRECATED=out-of-tree-build can break the initial setup, because this option gets applied to the original pip version (21.1 on Windows) instead of our pinned version. We should possibly ignore PIP_* variables or all environment variables during this initial setup for the pinned versions of things.

See pybind/cmake_example#66

Example of problematic line:

cibuildwheel/cibuildwheel/macos.py

Line 232 in f717468

env=env,

Build log

https://github.com/pybind/cmake_example/runs/4346290813?check_suite_focus=true

CI config

https://github.com/pybind/cmake_example/blob/c06fc016af76bd0f6a637705afc5d4cbfa554f5a/.github/workflows/wheels.yml#L49

[joerick]

Hmm yes, I can see the argument. We could also defer applying CIBW_ENVIRONMENT until after our internal tools install? Not sure which is preferable though.

[henryiii]

It would be simpler to defer till after the internal tools install, since we set a few pip variables (like the version check).