Summary:
`ysql_enable_packed_row` was an NewInstalOnly AutoFlag and incorrectly made to a `LocalPersisted` AutoFlag in d8acbed136c5deb18244018dafb5ea5102c174cb/D41634
This changes switches it to the correct `External` class.

Fixes #25881
Jira: DB-15188

Test Plan: Jenkins

Reviewers: rthallam

Reviewed By: rthallam

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D41692