fix: stop build variables leaking into the test environment - #3008

Summary

The test phase reused the build environment, so build-specific variables leaked into tests: _PYTHON_HOST_PLATFORM, ARCHFLAGS and SDKROOT on macOS, PYTHON_VERSION and PYTHON_ARCH on Windows. The test base environment is now rebuilt from the host environment plus the user environment option, per the approach in #1863.

Changes

• macos.py, windows.py: build test_base_env from os.environ plus build_options.environment instead of reusing the build env.
• CIBUILDWHEEL_BUILD_IDENTIFIER is preserved explicitly (all backends set it by design; the old test env carried it through the leak).
• linux.py deliberately untouched: nothing build-specific lives in its env, and host variables must never enter the container base env.

Testing

• No new unit test: the path needs real platform builders (a trivial helper test proves nothing). Existing integration tests exercise the test phase on CI runners.
• 466 env/platform unit tests pass locally. ruff check plus format (pinned rev), mypy strict on 3.11 plus 3.14, pylint 10/10, codespell clean.

Closes #1863