lib/file: fix MkdirAll after go1.21.4 stdlib update

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

lib/file/mkdir_windows.go

@@ -53,7 +53,7 @@ func MkdirAll(path string, perm os.FileMode) error {
 				j--
 			}
 			if j > 1 {
-				if path[:j-1] != `\\?\UNC` {
+				if path[:j-1] != `\\?\UNC` && path[:j-1] != `\\?` {
 					// Create parent.
 					err = MkdirAll(path[:j-1], perm)
 					if err != nil {