Is it free for workers to eat in the canteen during the epidemic prevention and control period at the construction site?

No specific regulations. It depends on how the employer decides.

This has nothing to do with the epidemic. Now is the era of market economy. Some units eat in the employee canteen for free, purely as employee benefits and give profits to employees. Some units only charge a nominal fee for meals. Some units outsource their canteens and earn employees' money from the canteens.