Build with the 3.0 dotnet SDK

[ericstj]

We should update the SDK we build with and clean up workarounds that were in place due to being on an old one.

Here are the workarounds I recall:
https://github.com/dotnet/corefx/blob/a680b92d4c7c1d99d9facf2b7eca8dd33ee87515/external/runtime/runtime.depproj#L53-L57
https://github.com/dotnet/corefx/blob/d07ba0a0026aa4effe9cc24ec5c1ed12e08c5d8d/Directory.Build.props#L145-L149

[ericstj]

@ViktorHofer I know you started looking at this when you were working on the global tools. Did you want to take this or update the issue with your findings?

[ViktorHofer]

Yeah I'm happy to take this. Will probably also require some minor arcade updates.

[ViktorHofer]

Related: dotnet/arcade#1907 & dotnet/arcade#1906