I really doubt (but don’t know for a fact) that it makes more than a few pennies of difference between having the AC on or off if the engine is idling anyway. When you are idling, most of the gas is wasted just keeping the engine going. Adding a small load to that probably doesn’t change much.
In any event, the treatment for the resulting heatstroke will cost far more than you would save. Just think about how much gas the ambulance would burn.