In ths security related issue the go1.21.4 stdlib changed the parsing
of volume names on Windows.
https://github.com/golang/go/issues/63713
This had the consequences of breaking the MkdirAll tests which were
looking for specific error messages which changed and using invalid
paths.
In particular under go1.21.3:
filepath.VolumeName(`\\?\C:`) == `\\?\C:`
But under go1.21.4 it is:
filepath.VolumeName(`\\?\C:`) == `\\?`
The path `\\?\C:` isn't actually a valid Windows path. I reported this
as a FYI bug upstream - I'm not expecting it to be fixed.
See: https://github.com/golang/go/issues/64101