Clear packages in config (#6544)

Author: stevejgordon

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="NuGet official package source" value="https://api.nuget.org/v3/index.json" />
 		<add key="Versioned Clients" value="https://f.feedz.io/elastic/all/nuget/index.json" />
 	</packageSources>