Add tools feed to NuGet.config

Workaround Microsoft/msbuild#2914

NuGet.config

@@ -2,6 +2,10 @@
 <configuration>
   <packageSources>
     <clear />
-    <!-- Restore sources should be defined in build/sources.props. -->
+    <!--
+      Restore sources should be defined in build/sources.props.
+      The only allowed feed here is myget.org/aspnet-tools which is required to workaround https://github.com/Microsoft/msbuild/issues/2914
+    -->
+    <add key="myget.org aspnetcore-tools" value="https://dotnet.myget.org/F/aspnetcore-tools/api/v3/index.json" />
   </packageSources>
 </configuration>