Enable MSVC build of Python bindings on GitHub Actions

Our GitHub Actions Windows build currently excludes the Python bindings because the compiler takes over 40 minutes to compile [functions.cc](https://github.com/deepmind/mujoco/blob/main/python/mujoco/functions.cc) and [structs.cc](https://github.com/deepmind/mujoco/blob/main/python/mujoco/structs.cc).

We have posted an [issue report with Microsoft](https://developercommunity.visualstudio.com/t/MSVC-19-takes-a-very-long-time-to-compil/10150657). If/when they fix the issue, we should start building and testing the Python bindings on MSVC.

Note that our official Python bindings releases for Windows are currently built with LLVM. Benchmarking in early 2022 showed that the LLVM-built binaries seem to be faster than MSVC-built ones anyway, so we are likely to continue to use LLVM for official releases regardless of the outcome of this issue.