Pres. Clinton today will tell the federal government to hop to it when he issues an executive order calling on government agencies to use less energy and cut greenhouse gas emissions 30 percent from 1990 levels by 2010. The U.S. government is the world’s largest energy consumer, spending more than $8 billion a year. Federal agencies will have to cut energy use for lighting, heating, and cooling, aiming for a reduction of 35 percent from 1985 levels by 2010; they are already halfway toward meeting that goal.