Change bootstrap framework CI leg to be consistent with SDK

[jjonescz]

Change the bootstrap framework CI leg to more closely follow what the .NET SDK is now doing (with the Microsoft.Net.Sdk.Compilers.Toolset package). Essentially set /p:RoslynTargetsPath=...\net472. That will help ensure our test coverage catches any authoring mistakes we make in roslyn.

See dotnet/sdk#41951 (comment).