Acererak has a legendary action to cast any of his at-will spells (any spell he has prepared of level 3 or lower). One of the spells in that list is
Animate Dead,
which normally has a casting time of 1 minute. Can he use that legendary action to cast it, accelerating its casting speed to "1 legendary action" instead of "1 minute", in the same way "1 action" is accelerated to "1 legendary action" normally?