nixos/etc: fix payload in build-composefs-dump for the file case

                        size=os.stat(source).st_size,

                        filetype=FileType.file,

                        mode=mode,

                        payload=target,

                        # payload needs to be relative path in this case

                        payload=target.lstrip("/"),

                    )

            paths[target] = composefs_path

            add_leading_directories(target, attrs, paths)