Amazon.com Inc., an early backer of Rivian Automotive, now owns about a fifth of the electric-truck maker, the e-commerce giant disclosed in a quarterly filing on Friday. Ford Motor Co., which has a strategic relationship with Rivian, removed a representative from the startup’s board earlier this month. At the end of the third quarter, Amazon had an equity interest of about 20 percent in Rivian, according to the filing. Amazon on Thursday reported lackluster quarterly results and warned that a tightening labor market and rising costs would hurt its holiday-quarter results.