entc/gen: update mutation-id on post mutation

entc/gen/template/dialect/sql/create.tmpl

@@ -120,6 +120,7 @@ func ({{ $receiver }} *{{ $builder }}) Save(ctx context.Context) ([]*{{ $.Name }
 						}
 					}
 				}
+				mutation.{{ $.ID.BuilderField }} = &nodes[i].{{ $.ID.StructField }}
 				mutation.done = true
 				if err != nil {
 					return nil, err