Fixup feeds (#437)

Co-authored-by: Matt Mitchell <mmitche@microsoft.com>

NuGet.config

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