Workers assembling General Motors Co.’s Tesla fighter are taking a month off this summer amid lukewarm demand, a sign American car buyers are showing little interest in vehicles that rely solely on battery power to get from Point A to B. The Detroit auto giant’s plant in Orion Township, Mich., began producing the Chevrolet Bolt late last year, with output hitting more than 15,000 vehicles before the company’s typical summer shutdown at the start of July. That break usually spans two weeks, but workers at the Bolt…