[main] Clear disabled package sources

- remove remaining way user NuGet.config can infect builds

Author: dougbu

NuGet.config

@@ -13,5 +13,7 @@
     <!-- Used for the SiteExtension 3.1 bits that are included in the 5.0 build -->
     <add key="dotnet31-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3.1-transport/nuget/v3/index.json" />
   </packageSources>
-  <disabledPackageSources />
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>