build: fix gocritic lint issue assignop

lib/file/mkdir_windows.go

@@ -42,7 +42,7 @@ func MkdirAll(path string, perm os.FileMode) error {
 			// In extended-length form without trailing slash ("\\?\C:"), os.Stat
 			// and os.Mkdir both fails. With trailing slash ("\\?\C:\") works,
 			// and regular paths with or without it ("C:" and "C:\") both works.
-			path = path + string(os.PathSeparator)
+			path += string(os.PathSeparator)
 		} else {
 			// See if there is a parent to be created first.
 			// Not when path refer to a drive's root directory, because we don't