[master] Clear disabled package sources (#3188)

- remove remaining way user NuGet.config can infect builds

NuGet.config

@@ -13,5 +13,7 @@
     <!-- Used for the Rich Navigation indexing task -->
     <add key="richnav" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-buildservices/nuget/v3/index.json" />
   </packageSources>
-  <disabledPackageSources />
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>