Cargo xwin configutation to download sdks and headers for target arch only.

[kivsiak]

I need to build x86_64 library.

maturin build --release --target x86_64-pc-windows-msvc -i python3.10

All going fine. A got my wheel. But when i deploy it to CI i found my builds are extremely slow. Investigating i found that xwin download and splat ALL possible variants of headers and sdk. While i need only x86_64.

I found that XWIN_VARIANT, XWIN_ARCH, as other xwin env variables does nothing.

There should be way to narrow download arch according build target

[messense]

Pull requests are welcome.

[messense]

I found that XWIN_VARIANT, XWIN_ARCH, as other xwin env variables does nothing.

These env vars works now thanks to https://github.com/PyO3/maturin/pull/1961