RepoToolset fails to load the correct compiler in WPF projects

The toolset uses a `` to reference Microsoft.Net.Compilers:

https://github.com/dotnet/roslyn-tools/blob/f093fd26f99e3e4aee739996eebb50fb3de15c0c/sdks/RepoToolset/tools/Compiler.props#L10

This feature assumes that build extensions included in the package will be used during the build. However, for the **.tmp_proj** created for WPF projects, this is not the case. The underlying bug is NuGet/Home#5894.

The workaround is to explicitly import the NuGet build files when building the WPF temporary project, as shown in dotnet/roslyn-sdk#94.