add msbuild feed

Author: enricosada

tests/FSharp.Compiler.Service.Tests.netcore/NuGet.Config

@@ -2,5 +2,7 @@
 <configuration>
   <packageSources>
     <add key="fsharp-netcore-preview2" value="https://www.myget.org/F/fsharp-netcoresdk-preview2-betatest/api/v3/index.json" />
+    <!-- MSBuild -->
+    <add key="dotnet-buildtools" value="https://dotnet.myget.org/F/dotnet-buildtools/api/v3/index.json" />
   </packageSources>
 </configuration>