Restore interval for process prune task to SolidQueue.process_alive_threshold

I had moved it to 2 seconds to debug and committed that by mistake ^_^U

Author: rosa

lib/solid_queue/supervisor.rb

@@ -66,7 +66,7 @@ def start_forks
       end
 
       def launch_process_prune
-        @prune_task = Concurrent::TimerTask.new(run_now: true, execution_interval: 2.seconds) { prune_dead_processes }
+        @prune_task = Concurrent::TimerTask.new(run_now: true, execution_interval: SolidQueue.process_alive_threshold) { prune_dead_processes }
         @prune_task.execute
       end