More robust identification of the build OS in Microsoft.AspNetCore.App.Runtime.csproj

https://github.com/dotnet/aspnetcore/blob/a0b950fc51c43289ab3c6dbea15926d32f3556cc/src/Framework/App.Runtime/src/Microsoft.AspNetCore.App.Runtime.csproj#L101-L110

This hardcodes some assumptions that are valid in the CI legs, but may not be in other systems.

For example, if this project is being built on Linux arm64 hardware using an OS that uses musl - in other words, `linux-musl-arm64`, it will try and use the `linux-arm64` crossgen2 instead.

Another example, if this code is being built on Windows on arm64 (or really, any architecture that's not x64), it will use the wrong crossgen, `win-x64`, which will most likely fail to run on an Arm 64 machine.

See also this comment posted by @dougbu in https://github.com/dotnet/aspnetcore/issues/37374#issuecomment-938358023

@dougbu