LOS ANGELES—The Flying Buttress once glided across the oceans carrying vital commodities like grain to all corners of the world. Now it bears a different treasure: Paw Patrol Movie Towers, Batmobile Transformers, and Baby Alive Lulu Achoo dolls. The dry bulk cargo ship has been drafted into the service of retail giant Walmart, which is chartering its own vessels in an effort to beat the global supply chain disruptions that threaten to torpedo the retail industry’s make-or-break holiday season. “Chartering vessels is just one example of investments we’ve made to move products as quickly as possible,” said Joe Metzger, U.S. executive vice president of supply-chain operations at Walmart, which has hired a number of vessels this year. The aim is to bypass log-jammed ports and secure scarce ship space at a time when COVID-19, as well as U.S.-China trade ructions, equipment shortages, and extreme weather, have exposed the fragility of …