CI fixes (#1233)

* Mimic other repo configurations.

* Test simple package source.

* Revert 2nd attempt.

NuGet.config

@@ -5,7 +5,8 @@
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
-  <disabledPackageSources />
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>