dotnet format in SDK 9.0.200 throws FileNotFoundException for .slnx solutions

[ownerer]

Describe the bug

dotnet format ConsoleApp1.slnx fails, throwing a FileNotFoundException

To Reproduce

• Create any new .slnx solution, no need to write any code, just a default console app will do.
• run dotnet format ConsoleApp1.slnx in a terminal

Exceptions (if any)

Unhandled exception: System.IO.FileNotFoundException: The file 'ConsoleApp1.slnx' does not appear to be a valid project or solution file.
   at Microsoft.CodeAnalysis.Tools.Workspaces.MSBuildWorkspaceFinder.FindFile(String workspacePath)
   at Microsoft.CodeAnalysis.Tools.Workspaces.MSBuildWorkspaceFinder.FindWorkspace(String searchDirectory, String workspacePath)
   at Microsoft.CodeAnalysis.Tools.FormatCommandCommon.ParseWorkspaceOptions(ParseResult parseResult, FormatOptions formatOptions)
   at Microsoft.CodeAnalysis.Tools.Commands.RootFormatCommand.FormatCommandDefaultHandler.InvokeAsync(ParseResult parseResult, CancellationToken cancellationToken)
   at System.CommandLine.Invocation.InvocationPipeline.InvokeAsync(ParseResult parseResult, CancellationToken cancellationToken)

An issue was also raised here, but since this is the main repo apparently I'm opening a new one.

[ownerer]

Tagging format area owners as per the CODEOWNERS file: @sharwell @arunchndr