Growth is expected to be flat in holiday-season hiring by U.S. companies — including retailers — this year. That is in large part because many retailers have already hired permanent employees in earlier rounds and need fewer temporary hands this season, according to Andrew Challenger, vice president of business development at global outplacement firm Challenger, Gray & Christmas. During a recent appearance on the Knowledge at Wharton radio show, Challenger urged those who are looking for some of those temporary jobs to act quickly. “If you want to take advantage of this extra bump in hiring that happens at the end of the year, you’ve got to start now,” he said. “It takes a little while to get on the radar of potential employers. So anybody looking to be hired for any of those positions [must] jump on it as soon as [they] can.” The Knowledge at Wharton show airs 10 a.m.-12 p.m. Eastern time Monday-Friday on Wharton Business Radio on SiriusXM channel 111.…