[main] Clear disabled package sources (#24224)

- remove remaining way user NuGet.config can infect builds

Author: dougbu

Diff for: NuGet.config

@@ -11,6 +11,7 @@
     <add key="richnav" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-buildservices/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
+    <clear />
     <!--  Begin: Package sources from dotnet-runtime -->
     <add key="darc-int-dotnet-runtime-cb5f173" value="true" />
     <!--  End: Package sources from dotnet-runtime -->