Can't run `dotnet format` using .NET 6

In NuGet.Client repo, if I try to run `dotnet format` tool using .NET 6 Preview, it gives me the following error:

```
PS C:\src\NuGet.Client.PR> dotnet tool restore
Tool 'dotnet-format' (version '4.1.131201') was restored. Available commands: dotnet-format

Restore was successful.
PS C:\src\NuGet.Client.PR> dotnet format --exclude submodules --verbosity diagnostic
The dotnet CLI version is '6.0.100-preview.6.21355.2'.
Using MSBuild.exe located in 'C:\Program Files\dotnet\sdk\6.0.100-preview.9.9999.9\'.
Formatting code files in workspace 'C:\src\NuGet.Client.PR\NuGet.sln'.
Loading workspace.
Could not load file or assembly 'System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.
```

My current workaround is to set in `global.json` a .NET 5 version.