gh-95174: Move WASIX logic into wasi-env (GH-95320)

wasi-env now sets WASIX flags. This allows us to control all build
parameter for wasm32-wasi buildbot from CPython repository.

Also export and improve SYSROOT parameter.
(cherry picked from commit 2833f3798dc3647e850b303a4d0fa00609a0ae9b)

Co-authored-by: Christian Heimes <christian@python.org>