Hawk Diesel
Adventurer
It's Con because you're resisting the effects of lightning coursing through your body.
I see and understand your reasoning, but requiring and a player to use an action in order to make a Constitution save bucks with the traditional format of 5e. Saving throws are always reflexive and don't require the player to use an action, while ability checks do require an action.
I would either change the wording to be the target creature makes a Con save at the end of each turn, or the target can attempt to escape using an action to make athletics or Acrobatics check.
I wouldn't recommend allowing an action to make a constitution ability check, since no one will have proficiency in a constitution check, and at most you'll have Bards and Champions with half proficiency.