Wholesale prices rose a brisk 0.6 percent in August as energy prices rose at their steepest clip since April but food costs posted only moderate gains, the government said Friday. If the August increase held for 12 straight months, inflation at the wholesale level would amount to 6.9 percent, more than triple the 2.2-percent 1987 price rise. Food prices one step short of the retail level were up a moderate 0.4 percent, the same as in July and another indication that the effects of the drought are moderating after steep rises in May and June.