Add missing <clear/> (#185)

NuGet.config

@@ -2,6 +2,7 @@
 <configuration>
   <!-- Only specify feed for Arcade SDK (see https://github.com/Microsoft/msbuild/issues/2982) -->->
   <packageSources>
+    <clear/>
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
   </packageSources>