Add missing efcore feed.

Author: mmitche

NuGet.config

@@ -7,6 +7,7 @@
     <add key="darc-int-dotnet-runtime-7280473" value="https://pkgs.dev.azure.com/dnceng/internal/_packaging/darc-int-dotnet-runtime-72804738/nuget/v3/index.json" />
     <!--  End: Package sources from dotnet-runtime -->
     <!--  Begin: Package sources from dotnet-efcore -->
+    <add key="darc-pub-dotnet-efcore-6fc3e170" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-efcore-6fc3e170/nuget/v3/index.json" />
     <!--  End: Package sources from dotnet-efcore -->
     <!--End: Package sources managed by Dependency Flow automation. Do not edit the sources above.-->
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />