How long will Postmaster General Marvin T. Runyon hang on to his job? Not long, if his support on the Postal Board of Governors keeps eroding due to unhappiness with Runyon's performance. Robert Setrakian, a Reagan appointee, had been alone among the eight board members in seeking Runyon's ouster. Now, Setrakian has an ally. Clinton appointee Einar V. Dyhrkopp says he will vote to dump Runyon at the board's Aug. 29 meeting. The other governors are likely to keep him--but on a short leash. Dyhrkopp says he would be surprised if Runyon stays beyond yearend.