Sort field change without field change should mark schema modified

Closes #1789

rslib/src/notetype/schemachange.rs

@@ -69,6 +69,7 @@ impl Collection {
         if !ords_changed(&ords, previous_field_count) {
             if nt.config.sort_field_idx != previous_sort_idx {
                 // only need to update sort field
+                self.set_schema_modified()?;
                 let nids = self.search_notes_unordered(nt.id)?;
                 for nid in nids {
                     let mut note = self.storage.get_note(nid)?.unwrap();