I don’t think a new variable is needed. It looks like you can set CMAKE_OSX_ARCHITECTURES to $(ARCHS_STANDARD) if you want a universal build where available for macOS. CMake’s default behavior has always been to build for the native host architecture only. There was a fair amount of investigation and work that went into keeping that behavior with Xcode 12 and Apple Silicon to avoid various problems. See issue 20893 for the fairly detailed discussions and related information.
Depending on what version of macOS and Xcode you are running, you may also need to set CMAKE_OSX_DEPLOYMENT_TARGET to be able to run your built apps. I have the situation locally where I build with Xcode 12.3 on an intel running macOS 10.15. If I don’t specify CMAKE_OSX_DEPLOYMENT_TARGET, I end up building a valid binary that I can’t run on my machine.