Although the CMake documentation appears to require platform-specific config file locations, it caveats that with:

This is merely a convention, so all (W) and (U) directories are still searched on all platforms

Due to this, and because with not deploying as a framework/bundle thing on MacOS, we can just set the default location to share/arg_router which conveniently is what vcpkg wants.

It would be good to allow this location to be overridden by the user if required, so expose a cache variable too.

• Update install.cmake to use single location using a cache variable, defaulting to share/arg_router
• Update GHAs for new location