Create a setup-cmake action for both Mac and Windows

[Steelskin]

trafficstars

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.