[release/3.1] Clear disabled package sources (#30400)

- remove remaining way user NuGet.config can infect builds

Author: dougbu

NuGet.config

@@ -26,6 +26,7 @@
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
+    <clear />
     <!--Begin: Package sources managed by Dependency Flow automation. Do not edit the sources below.-->
     <!--  Begin: Package sources from dotnet-extensions -->
     <add key="darc-int-dotnet-extensions-945dd07" value="true" />