Build .NET Core 2.1 apps with msbuild

[twsouthwick]

We are currently using the latest-vs15-5 image, but in order to build .NET Core 2.1 apps, we need to have VS 15.7. We build with msbuild due to some projects (a VS extensions) requiring it, so we can't load our own .NET CLI. This is for Microsoft/dotnet-apiport