Investigate multibuild issue

[TomAugspurger]

On master we needed to set multibuild to 47f4b247be212315313ff72e3daafdff0cbc6c33.

multibuild devel (as of 217614c7bdc27ac8f5bb29dab0568594b8f4f352) was failing.

failed: https://dev.azure.com/pandas-dev/pandas-wheels/_build/results?buildId=30804&view=results
passed: https://dev.azure.com/pandas-dev/pandas-wheels/_build/results?buildId=30802&view=results

The python in install_run wasn't the test_venv python. Haven't bisected it yet, but based on the commit messages, these are candidates

| * be1744e - set and use PYTHON_EXE (12 days ago) <mattip>
| * 9133448 - use 'python -mpip' to work around macOS problems (13 days ago) <mattip>

[mattip]

I think multibuild needs to finish and commit matthew-brett/multibuild#315 in order to mitigate this.

[mattip]

@TomAugspurger could you give the latest devel branch a try? The PR that I think fixes this was merged.