python: honour PYTHONEXECUTABLE if set

[mjpieters]

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• Have you checked that there aren't other open pull requests for the same formula update/change?
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Proposed fix for issue #33963: if the PYTHONEXECUTABLE environment variable is set, do not alter sys.executable.

[fxcoudert]

@BrewTestBot test this please

[mjpieters]

I'm confused by these build errors. None of those projects could be affected by the PYTHONEXECUTABLE change, since linking doesn't care about the sys.executable path.

[SMillerDev]

@BrewTestBot test this please

[fxcoudert]

Let's try it again: @BrewTestBot test this please

[fxcoudert]

Thanks @mjpieters this is good to go.