Note that:
Aluminium is good conductor of heat because of its high thermal conductivity
The solar cooker cooks food with the heat is energy absorbed from sunlight. The aluminium foil is used as a reflector in the solar oven so as to gather/trap sufficient sunlight into the box inorder to be used by the cooker.
Therefore, the presence of aluminium/tin foil in the solar oven will help speed up the rate at which heat is trapped, hence the rate at which the food gets cooked.
More tin/ Aluminium foil means more reflection of sunlight in the solar oven.