Unions representing federal employees are urging President-elect Bush and Congress to reject President Reagan's budget recommendation of a 2 percent pay raise for government workers. Advocates for federal retirees also attacked the spending plan Monday, saying it was unfair for Reagan to recommend that the retirees be denied a cost-of-living adjustment next year while awarding such an increase to Social Security recipients. Both groups said the principal ammunition in their fight against the Reagan budget proposals will be the 50 percent raise the president recommended for members of Congress, federal judges and other government executives. The unions also complained that Reagan recommeded a 3.6 percent pay raise for military personnel but only 2 percent for civilian workers.