ci: Build R3.WinUI3 on CI

[guitarrapc]

tl;dr;

I've confirm R3.WinUI3 can build/pack on WSL Ubuntu 22.04. Looking into CI error message, error caused on sandbox.WinUI3App1 not with R3.WinUI3.

• Enable R3.WinUI3 sln build.
• Disable sandbox/WinUI3App1 sln build.

Buil on CI

WinUI3 build has succeed.

https://github.com/Cysharp/R3/actions/runs/7969093258/job/21754351812#step:4:57

2024-02-20T05:57:38.0221985Z CSC : warning CS8002: Referenced assembly 'MonoGame.Framework, Version=3.8.1.303, Culture=neutral, PublicKeyToken=null' does not have a strong name. [/home/runner/work/R3/R3/src/R3.MonoGame/R3.MonoGame.csproj::TargetFramework=net6.0]
2024-02-20T05:57:38.0809628Z R3.WinUI3 -> /home/runner/work/R3/R3/src/R3.WinUI3/bin/Debug/net6.0-windows10.0.19041.0/R3.WinUI3.dll

Build R3.WinUI3 on WSL Ubuntu 22.04

$ dotnet pack -c Release ./src/R3.WinUI3 -p:Version=1.0.0 -o ./publish
MSBuild version 17.9.4+90725d08d for .NET
  Determining projects to restore...
  Restored /mnt/c/github/cysharp/R3/src/R3.WinUI3/R3.WinUI3.csproj (in 303 ms).
  1 of 2 projects are up-to-date for restore.
  R3 -> /mnt/c/github/cysharp/R3/src/R3/bin/Release/net6.0/R3.dll
  R3.WinUI3 -> /mnt/c/github/cysharp/R3/src/R3.WinUI3/bin/Release/net6.0-windows10.0.19041.0/R3.WinUI3.dll
  The package R3Extensions.WinUI3.1.0.0 is missing a readme. Go to https://aka.ms/nuget/authoring-best-practices/readme to learn why package readmes are important.
  Successfully created package '/mnt/c/github/cysharp/R3/publish/R3Extensions.WinUI3.1.0.0.nupkg'.