pkg/idtools: don't use system.MkdirAll() where not needed

On unix, it's an alias for os.MkdirAll, so remove its use to be
more transparent what's being used.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/idtools/idtools_unix.go

@@ -65,7 +65,7 @@ func mkdirAs(path string, mode os.FileMode, owner Identity, mkAll, chownExisting
 				paths = append(paths, dirPath)
 			}
 		}
-		if err := system.MkdirAll(path, mode); err != nil {
+		if err := os.MkdirAll(path, mode); err != nil {
 			return err
 		}
 	} else {