Set up Windows build in our CI

[lgritz]

Our CI doesn't actually build and test on Windows. This causes no end of despair.

How hard can it be? You don't really need to know anything about OSL internals or even C++ programming. It's mainly about wrangling builds and knowing how such things are done on Windows. Maybe it's even a good Dev Days project?

OpenImageIO's build and CI system is basically the same as OSL's, and it has Windows builds in its CI, so that should provide much of the template (and existence proof) for how this can be done. Probably the hardest part is that unlike OIIO, OSL will need LLVM installed as a dependency.