The growing use of digital price tags at Walmart and other big US retailers is stirring fresh anxiety that prices on groceries and other basic goods could be subjected to high-tech manipulation — and labor unions are looking to capitalize on the fears. Walmart said it is rapidly installing the tags — which can raise or lower the prices displayed on their tiny LED screens en masse with the click of a button — in all of its 4,600 US stores by the end of the year. The idea, Walmart says, is to