Fix bug where external sky maps built from older GCN notice

When updating an external event with info from a new GCN notice, we currently take the old event to create a new skymap: https://git.ligo.org/emfollow/gwcelery/blob/master/gwcelery/tasks/external_triggers.py#L115

Instead we should make sure we are grabbing the latest version of the external event after it's update.

Example of error: https://gracedb-playground.ligo.org/events/E223907/view/