Harvard’s striking cafeteria workers got another celebrity endorsement this week. Comedian Keegan-Michael Key, who’s in town shooting the Net­flix series “Friends From College,’’ showed support for the workers by holding one of their signs. Unite Here Local 26, the union that represents the 750 dining hall workers, posted a pic of Key on Facebook. Last week, a photo of Ben Stiller surfaced on social media showing the “Something About Mary’’ star holding a “Support the Strike’’ sign while on a break from shooting a movie in Harvard Square. Local 26 wants an increase in the annual minimum pay from $30,000 to $35,000, and no increase in health care premiums.