First off, understand what that fog on your lenses is. As you exhale through the mask, some of that air goes straight upwards. The water vapor droplets from your warm, exhaled breath condense on the cooler surface of the lenses, forming that fog.
Immediately before wearing a face mask, wash the spectacles with soapy water and shake off the excess. Then, let the spectacles air dry or gently dry off the lenses with a soft tissue before putting them back on. Now the spectacle lenses should not mist up when the face mask is worn.
This should come in handy not only when you're wearing a mask for COVID-19 protection--but when all of this is finally over and you're back in your shop, wearing a mask for dust protection. Gosh, that sounds like such a luxury right now.