`AcceptExistingOnly` for `FileInfo` accepts existing directories

[Coryf88]

Version 2.0.0 (Also affected 2.0.0-rc.2.25502.107, upgraded while creating issue)

A FileInfo argument/option with AcceptExistingOnly validation, considers an existing directory as valid.

[KalleOlaviNiemitalo]

I think it was broken by #2077 which added the Directory.Exists check here:

command-line-api/src/System.CommandLine/ArgumentValidation.cs

Line 187 in e292617

else if (checkFile && !Directory.Exists(token.Value) && !File.Exists(token.Value))

Prior to that, it would only have used File.Exists.