Create a setup-build action for both Mac and Windows

We need to do different setup for Mac and Windows hosts:
* On Windows, we use `compnerd/gha-setup-vsdevenv@main` to setup Visual Studio (which includes CMake and ninja as part of the CMake installation)
* On Mac, we `seanmiddleditch/gha-setup-ninja@master` to setup ninja, since CMake is already installed on the host.

We should have a single action to do both, tentatively called a `setup-cmake` action that we would depend on.