No, there isn't a supermarket directly on-site at the Circuit de la Sarthe during the 24 Hours of Le Mans, but there are food stands and restaurants, and you'll need to travel outside the circuit to access a supermarket.
The circuit has numerous food stands and restaurants to cater to the crowds.
To stock up on groceries, you'll need to go to a supermarket located outside of the circuit.
There are a Carrefour CARREFOUR (North of Tetre Rouge) 309 Avenue Georges Durand, F-72019 Le Mans and a SUPER U at ARNAGE (South West of circuit) Centre Commercial Rive Sud, Boulevard Pierre Lefaucheux, F-72230 Arnage
The Mulsanne village, near the circuit, has a reasonable supermarket, shops, and bars within walking distance.