python@3.9: avoid shelling out to brew

[carlocab]

This breaks builds inside superenv. See

• libcython: use Python's libexec/"bin" directory #107660
• libpython-tabulate: use Python's libexec/"bin" #107662
• libxml2: use explicitly versioned Python in test #107663
• numpy: use Python's libexec/"bin" #107664
• pillow: use Python's libexec/"bin" #107665
• Add support for Homebrew on Python 3.9 pypa/distutils#155
• Fails when the brew command is not available pypa/distutils#158

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[carlocab]

Still looking at backporting the 3.10 patch. Is this a suitable workaround in the meantime, @Bo98?

[carlocab]

Merging to unblock a bunch of other PRs.

[Bo98]

Yeah seems fine as a temp fix.