Don't try to grab sky map from Fermi if already present

Looking at the logs of https://gracedb.ligo.org/events/E451636/view/ we see that the identical sky map was downloaded multiple times, wasting computing resources both with uploading the sky map but also recalculating the joint FAR and creating corresponding combined sky maps.

This could be fixed by applying a label when uploading this type of sky map or doing a check for the latest filename here