update nuget.config (#1207)

Author: xseeseesee

NuGet.Config

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear /> <!-- ensure only the sources defined below are used -->
     <add key="NugetOfficialV3" value="https://api.nuget.org/v3/index.json" />
     <!-- To Enable LocalFeed for testing uncomment the following line -->
     <!-- <add key="Local" value="tools/LocalNugetFeed" /> -->