i think the tram from the airport now goes to jean médicin in nice, so i would take that if possible (though i'm not sure how late the new tram runs) -- i don't think it's in a rough area to walk to st augustin but it seems out of the way to just take the train one stop to nice, i have walked it plenty (and now take the tram) but i usually am coming from or going to monaco or menton
given your late arrival time i would also recommend uber into nice, should be ~€20 depending on where you're going...