WASHINGTON — President Donald Trump signed an executive order Monday that temporarily allows dyed, off-road diesel on the highway and defers the federal excise tax on that fuel through Dec. 31, a step the White House says is meant to cut costs for farmers, truckers, and other diesel users. The order, titled “Emergency Tax Relief on Diesel Fuel,” directs the Treasury secretary, in consultation with the secretary of war, to determine within five days whether relief is authorize