Being required to wear a mask has nothing to do with your religious objections to getting a flu shot. It has evertythng to do with your not getting the shot itself. In other words, it's not the reason, it's the result. In order to protect your fellow co-workers and others, you can be required to take precautions such as wearing a mask while working your schedued shift. The fact is that most employment relationships are "at will". This means that an employer can set the conditions of work much as they see fit, absent some kind of legally actionable discrimination (which does not appear to apply to your situation).