FileIO bug for Test when opening new file for writing

[janigbg]

When opening a new file for writing using the FileIO Test implementation I get a FileNotFoundException.
According to Microsoft docs the File.OpenWrite() method used should allow that:
https://learn.microsoft.com/en-us/dotnet/api/System.IO.File.OpenWrite?view=net-7.0#remarks

The FileMode is currently set to Open instead of OpenOrCreate in the test implementation.

Repro (this should not fail):

var effect =
    File<Runtime>.openWrite("c:\\file")
    | Consumer.mapM<Runtime, Stream, Unit>(_ => unitEff);

var result = await effect.RunEffect().Run(Runtime.New());
result.ThrowIfFail();