Use new dotnet-tools feed

- still need additional feeds like aspnet-extensions because #14425 is blocked

Author: dougbu

NuGet.config

@@ -14,6 +14,7 @@
     <add key="aspnet-extensions" value="https://dotnetfeed.blob.core.windows.net/aspnet-extensions/index.json" />
     <add key="aspnet-entityframeworkcore" value="https://dotnetfeed.blob.core.windows.net/aspnet-entityframeworkcore/index.json" />
     <add key="aspnet-aspnetcore-tooling" value="https://dotnetfeed.blob.core.windows.net/aspnet-aspnetcore-tooling/index.json" />
+    <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
     <add key="grpc-nuget-dev" value="https://grpc.jfrog.io/grpc/api/nuget/v3/grpc-nuget-dev" />
     <add key="roslyn" value="https://dotnet.myget.org/F/roslyn/api/v3/index.json" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />