There is actually no mandatory lunch break in France at all. If you work more than 6 hours in a day, your employer has to give you a 20-minutes break (for lunch or otherwise), which is not counted as part of your working time. A collective bargaining agreement can provide for other rules and most employees take much longer lunch breaks.
Legally, you do not have to spend your break on site and people occasionally run errands, practice sports or go to a restaurant during this time. In smaller towns, it's not uncommon to go back home for lunch. But if you live further away and that's not practical for you, an 8-hour working day will indeed force you to stay at or near your workplace for 9+ hours.
Finally, if your job involves clocking in and out, there will be more specific rules that are probably more relevant to you than what the law says.