roslyn: `C# Interactive` is broken in VS16.8 preview5

Version Used: VS16.8 preview5

🔗 Also reported at AB#1231577

Steps to Reproduce:

1. Open C# Interactive;
2. Output any non-ascii string like ("测试");
3. #reset core(#reset 32 or #reset 64 works well)

Expected Behavior:

1. non-ascii string should be supported;
2. Switch to core runtime without error;

<del> This only happen on one of my PC, not sure what’s the difference between them, and FUSLOGVW.exe show nothing for this </del> See https://github.com/dotnet/roslyn/issues/48874#issuecomment-715654253 instead if not reproducable in your PC for extra steps;

Actual Behavior:

1. Encoding not work

2. Failed to load System.IO.Pipelines, Version=4.0.2.1

> #reset core
Resetting execution engine.
Failed to launch 'c:\program files (x86)\microsoft visual studio\2019\preview\common7\ide\commonextensions\microsoft\managedlanguages\vbcsharp\languageservices\InteractiveHost\Core\InteractiveHost64.exe' process (exit code: 1) with output: 
System.IO.FileNotFoundException: Could not load file or assembly 'System.IO.Pipelines, Version=4.0.2.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'. ϵͳ�Ҳ���ָ�����ļ���
File name: 'System.IO.Pipelines, Version=4.0.2.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'
   at StreamJsonRpc.JsonMessageFormatter..ctor(Encoding encoding)
   at StreamJsonRpc.JsonMessageFormatter..ctor()
   at Microsoft.CodeAnalysis.Interactive.InteractiveHost.CreateRpc(Stream stream, Object incomingCallTarget)
   at Microsoft.CodeAnalysis.Interactive.InteractiveHost.Service.RunServerAsync(String pipeName, Int32 clientProcessId, Func`2 invokeOnMainThread)
   at Microsoft.CodeAnalysis.Interactive.InteractiveHostEntryPoint.Main(String[] args)

Not sure if the 2 error is related.