Cannot use 'dotnet build' to build the repo

[MiYanni]

Related: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1495770
Related: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1522781
(Potentially) Related: dotnet/msbuild#4704

Normally, any SDK-style project that msbuild can build, the dotnet CLI using dotnet build can also build. This is because under-the-hood, the dotnet CLI is calling msbuild. The dotnet CLI also exposes this directly with the dotnet msbuild command. However, these commands are calling a version of msbuild called msbuild core. There are some slight differences between msbuild and msbuild core. The difference that concerns us is a set of types in the msbuild SDK that msbuild core doesn't have. These are the 'XamlTypes' via Microsoft.Build.Framework.XamlTypes. An existing issue against MSBuild details this difference. Once msbuild core contains XamlTypes, we should verify we can properly build the repo using dotnet build.

Additionally, once the repo can build using dotnet build, our build.cmd can be simplified to call dotnet build instead of requiring a separate mechanism to locate msbuild. The dotnet CLI is always added to the system PATH when the dotnet SDK is installed, so we don't need any additional lookup to use this.

Error Message

[robertmclaws]

Would be great if we could get this fixed