Use dotnet-public instead of nuget.org

Author: MattGal

NuGet.config

@@ -3,7 +3,7 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+    <add key="dotnet-public" value="https://dnceng.pkgs.visualstudio.com/public/_packaging/dotnet-public/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
     <clear />