Respect _PYTHON_HOST_PLATFORM env variable

[ryanking13]

Adds a check for _PYTHON_HOST_PLATFORM variable when producing the best wheel tag. I was guessing that this file should be the place I should put this, but please let me know if I am editing the wrong file.

Resolves: #920

[LecrisUT]

Looks generally ok, but do not have the context of how this is defined and by what, but assuming everything is fine on #920 side, I don't see a reason why not to add it. Thanks :) . Do you think you can fit in a note in https://scikit-build-core.readthedocs.io/en/latest/guide/crosscompile.html?

[ryanking13]

Looks generally ok, but do not have the context of how this is defined and by what, but assuming everything is fine on #920 side, I don't see a reason why not to add it. Thanks :)

I think this is an undocumented variable that is used by several packages, including sysconfig, cibuildwheel, auditwheel, setuptools (link1, link2). Originally used from distutils I guess.

[ryanking13]

cc: @henryiii