[release/5.0] Clear disabled package sources (#30401)

- remove remaining way user NuGet.config can infect builds

Author: dougbu

NuGet.config

@@ -19,6 +19,7 @@
     <add key="dotnet31-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3.1-transport/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-runtime -->
     <add key="darc-int-dotnet-runtime-f27d337" value="true" />