Cross compilation broken for occt-sys

[DSchroer]

If you try to cross compile using the builtin occt-sys. It will always compile OCCT for the HOST architecture rather than the TARGET architecture.

This is due to it being a build-dependency.

opencascade-sys: built for TARGET
    build.rs: built/run for HOST with env TARGET=TARGET
        occt-sys: built for HOST (since its a dep of build.rs)
            build.rs: build for HOST with env TARGET=HOST