Use box_ready for $&, $`, $\', $+

These variables have rb_gvar_readonly_setter, so box_ready is sufficient.
Only $~ needs box_dynamic due to its custom match_setter.

Author: afurm

re.c

@@ -4901,10 +4901,10 @@ Init_Regexp(void)
     rb_gvar_ractor_local("$'");
     rb_gvar_ractor_local("$+");
     rb_gvar_box_dynamic("$~");
-    rb_gvar_box_dynamic("$&");
-    rb_gvar_box_dynamic("$`");
-    rb_gvar_box_dynamic("$'");
-    rb_gvar_box_dynamic("$+");
+    rb_gvar_box_ready("$&");
+    rb_gvar_box_ready("$`");
+    rb_gvar_box_ready("$'");
+    rb_gvar_box_ready("$+");
 
     rb_define_virtual_variable("$=", ignorecase_getter, ignorecase_setter);