fix: do not use random ports for pg.listen to avoid flakiness

From CI logs:

i1  | 2024-09-18 16:39:06.953 [35812] main/111/.proc_update_instance V> done waiting for applied index 8, current: 8
i1  | 2024-09-18 16:39:06.954 [35812] main/104/interactive I> self-activated successfully
i1  | 2024-09-18 16:39:06.954 [35812] main/104/interactive I> starting postgres server at ("127.0.0.1", 5734)...
i1  | CRITICAL: io error: Address already in use (os error 98)
i1  | 2024-09-18 16:39:06.954 [35812] main/104/interactive C> io error: Address already in use (os error 98)