Update NuGet 3.4.3

Author: veblush

tools/nuget/NuGet.Config

@@ -3,6 +3,9 @@
   <solution>
     <add key="disableSourceControlIntegration" value="true" />
   </solution>
+  <packageSources>
+    <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
+  </packageSources>
   <activePackageSource>
     <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
   </activePackageSource>