Don't worry and good luck on the interview!
This is not the day to change your diet. You will be sharper and more able to focus with a good diet of protien and limited carbs. If you want a cheat day, do it the day after. Most people are more concerned about what they eat, so they won't pay attention to you unless you bring it up. Just say you avoid bread for "medical reasons" if confronted. There ARE plenty of people (myself included) who have gluten and wheat intolerances.

Happy interviewing!!