Revert "cryptography-cffi: substitute include path from target sysroo…

…t in cross builds (pyca#9105)"

The original code was right all along: it uses the official API for
obtaining header locations, and it is on the build environment
to ensure python supplies that in cross-build scenarios
(another option is to apply a custom patch, however any such patch
is not eligible for upstream submission due to its specificity).

Further info: pyca#9129

src/rust/cryptography-cffi/build.rs

@@ -49,7 +49,7 @@ fn main() {
     println!("cargo:rustc-cfg=python_implementation=\"{}\"", python_impl);
     let python_include = run_python_script(
         &python,
-        "import sysconfig; print(sysconfig.get_config_var('INCLUDEPY'), end='')",
+        "import sysconfig; print(sysconfig.get_path('include'), end='')",
     )
     .unwrap();
     let openssl_include =