Fix C# GitHub Action script

[huan]

Current it has errors:

wechaty/puppet/base.proto : error : File not found. [D:\a\grpc\grpc\csharp\Wechaty.Grpc\Wechaty.Grpc.csproj]

It might be related to the proto/ folder change: our protocol buffers files are under proto/ folder now.

@wechaty/dotnet Could you please have a look at it? It would be great if we can fix it because it's the only failing CI for now.

[huan]

The file 'D:\a\grpc\grpc\polyglot\csharp\Wechaty.Grpc\bin\Release\netstandard2.0\Wechaty.Grpc.dll' to be packed was not found on disk.

See: https://github.com/wechaty/grpc/runs/3799440509?check_suite_focus=true