WASHINGTON — A federal appeals court on Friday rejected a Trump administration finding that the active ingredient in the weed killer Roundup does not pose a serious health risk and is “not likely” to cause cancer in humans. The California-based 9th U.S. Circuit Court of Appeals ordered the Environmental Protection Agency to reexamine its 2020 finding that glyphosate did not pose a health risk for people exposed to it by any means — on farms, yards or roadsides or as residue left on food crops. Glyphosate is the active ingredient in Roundup, the most widely used herbicide in the world.