Supplement to #6781

`mrb_str_dup()` always duplicates string objects in an unfrozen state, and the class is also set.
Therefore, it can be observed and modified from the Ruby side using the `ObjectSpace.each_object` method.

By using `mrb_str_dup_frozen()`, unnecessary duplication can be avoided, and modifications to the string can also be prevented.

mrbgems/mruby-sprintf/src/sprintf.c

@@ -386,7 +386,7 @@ mrb_str_format(mrb_state *mrb, mrb_int argc, const mrb_value *argv, mrb_value fm
      buffer, so this is O(1); String#replace on the original goes
      through str_replace which decrements the shared refcount, leaving
      our copy's buffer intact. */
-  fmt = mrb_str_dup(mrb, fmt);
+  fmt = mrb_str_dup_frozen(mrb, fmt);
   p = RSTRING_PTR(fmt);
   end = p + RSTRING_LEN(fmt);
   blen = 0;