In order to use CMake + Ninja + MSVC (The Visual Studio Compiler) you need to startup a developer command prompt.
Which under the hood is really calling vcvarsall.bat which is found in a visual studio installation.
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat
How can I do this via GitHub actions?
Currently we use https://github.com/ilammy/msvc-dev-cmd to setup the developer command prompt for our C++ projects. It works quite well and has a nice interface.
I'm not sure if this is the most idiomatic solution but it has been working quite nicely so far.