Remove build specific environment variables from the test environment

[mayeut]

Description

macOS builds are using multiple environment variables specific to the build environment:
MACOSX_DEPLOYMENT_TARGET, _PYTHON_HOST_PLATFORM, ARCHFLAGS, SDKROOT

Those are propagated to the test environments. They should probably be removed from those.

see #1856 (comment) and the 2 following comments for more context.

Build log

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CI config

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[henryiii]

While MACOSX_DEPLOYMENT_TARGET makes sense, the others might cause it to be harder to build wheels in the test step if wheels are missing from test dependencies. Not the best idea to build here, in general, but it may currently work because the correct variables are set for building.

[mayeut]

Maybe we should adapt them all then ?

If we're thinking that we might break some edge case that requires building from sources by removing any of those environment variable then IMHO we should update them all:

• MACOSX_DEPLOYMENT_TARGET to the current running OS (this one is already done but was meant to be temporary, required by uv for now)
• _PYTHON_HOST_PLATFORM/ARCHFLAGS to the current running test architecture as it's more likely to fail an universal2 build than a specific one. In fact, that was the issue when building matplotlib from sources in feat: build[uv] #1856 (comment), it might have failed later on of course.