Fix: Grease Pencil: Missing call to finish() writing layer attributes

The `GSpanAttributeWriter` was missing a call to `finish()` to ensure
the changes are tagged and saved.

source/blender/blenkernel/intern/grease_pencil.cc

@@ -3789,6 +3789,7 @@ blender::bke::greasepencil::Layer &GreasePencil::duplicate_layer(
     bke::GSpanAttributeWriter attr = attributes.lookup_for_write_span(iter.name);
     GMutableSpan span = attr.span;
     span.type().copy_assign(span[*duplicate_layer_idx], span[numLayers]);
+    attr.finish();
   });
 
   this->update_drawing_users_for_layer(*new_layer);