The work environment for the production of face masks depends on the intended use and declarations that you want to put on your medical device. If you want to sell sterile face masks, then you need sterilization facilities. In that case, each mask will be packed in a separate pouch and will be class Is.
If you do not want it to be sterile, then it is class I. There is no direct requirement in the standard to produce those masks in the cleanroom area. However, in ISO 13485:2016 in requirement 6.4.1 Work environment is stated that organization must be defined and document requirements needed to achieve conformity to product requirements. So, it is up to you how you will decide.
For more information regarding the work environment, please see the following links: