GitHub Actions keep failing, .NET 6 related?

Describe the bug
The build using GitHub Actions fails while doing the build-windows part. It might be .NET 6 related but I am not really sure as I have never worked with Actions and don't fully understand whats going on.

To Reproduce
Steps to reproduce the behavior:

1. Download latest Remotely Installer
   wget https://github.com/lucent-sea/Remotely/releases/latest/download/Remotely_Server_Installer
2. Make it +x
   chmod +x Remotely_Server_Installer
3. Run it with sudo
   sudo ./Remotely_Server_Installer
4. Choose not to download per-built package
5. Choose /var/www/remotely/ as directory
6. Entered webserver url. It is a local webserver, only accessible from LAN (domain.local)
7. Use Caddy on Ubuntu
8. Entered GitHub username and PAT
9. Create latest official release

Performing server install.  GitHub User: DaniW42.  Server URL:
http://remote.beautypark.local/.  Installation Directory: /var/www/remotely/.
Web Server: UbuntuCaddy.  Create New Build: True.  Git Reference:
v2021.08.26.1335

Now a infinite loop of Waiting for GitHub build completion. and Get artifacts response status code: OK starts.

But, GitHub Actions throws an error an the step Execute unit tests and stops build.

Run dotnet test
  dotnet test
  shell: C:\Program Files\PowerShell\7\pwsh.EXE -command ". '{0}'"
  env:
    Solution_Name: Remotely.sln
    Configuration: Release
    PfxBase64: 
    PfxKey: 
    ServerUrl: http://remote.beautypark.local/

Welcome to .NET 5.0!
---------------------
SDK Version: 5.0.403

Telemetry
---------
The .NET tools collect usage data in order to help us improve your experience. It is collected by Microsoft and shared with the community. You can opt-out of telemetry by setting the DOTNET_CLI_TELEMETRY_OPTOUT environment variable to '1' or 'true' using your favorite shell.

Read more about .NET CLI Tools telemetry: https://aka.ms/dotnet-cli-telemetry

----------------
Installed an ASP.NET Core HTTPS development certificate.
To trust the certificate run 'dotnet dev-certs https --trust' (Windows and macOS only).
Learn about HTTPS: https://aka.ms/dotnet-https
----------------
Write your first app: https://aka.ms/dotnet-hello-world
Find out what's new: https://aka.ms/dotnet-whats-new
Explore documentation: https://aka.ms/dotnet-docs
Report issues and find source on GitHub: https://github.com/dotnet/core
Use 'dotnet --help' to see available commands or visit: https://aka.ms/dotnet-cli
--------------------------------------------------------------------------------------
  Determining projects to restore...
Error: C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [D:\a\Remotely\Remotely\Desktop.Core\Desktop.Core.csproj]
Error: C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [D:\a\Remotely\Remotely\Shared\Shared.csproj]
Error: C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [D:\a\Remotely\Remotely\Agent\Agent.csproj]
Error: C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [D:\a\Remotely\Remotely\Desktop.XPlat\Desktop.XPlat.csproj]
Error: C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [D:\a\Remotely\Remotely\Desktop.Win\Desktop.Win.csproj]
Error: C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [D:\a\Remotely\Remotely\Tests\Tests.csproj]
Error: C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [D:\a\Remotely\Remotely\Tests.LoadTester\Tests.LoadTester.csproj]
Error: C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [D:\a\Remotely\Remotely\Server.Installer\Server.Installer.csproj]
Error: C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [D:\a\Remotely\Remotely\Server\Server.csproj]
Error: Process completed with exit code 1.

Remotely Version
Server: v2021.08.26.1335 Release
Agent: didn't got that far

Expected Behavior
I think it should have built the agents and downloaded them to my local server.

Screenshots

Desktop (please complete the following information):
not relevant

Smartphone (please complete the following information):
not relevant

Additional Context
I retried the build process again but it keeps failing at the same stage.

Start with the GitHub Actions workflow at the Execute unit tests step and inspect how the SDK is selected before dotnet test runs. Compare that setup with the .NET 6 target frameworks reported in the build log. Done means the workflow can restore and run dotnet test successfully for the solution.