try to fix "too many open files" error

asyncutils.nim

@@ -34,8 +34,10 @@ proc asyncExecCmd*(command: string): Future[string] =
     (result, exitCode) = execCmdEx(command)
     if exitCode != 0:
       failureEvent.trigger()
+      successEvent.unregister()
     else:
       successEvent.trigger()
+      failureEvent.unregister()
   let flowVar = spawn execCmdBackground(successEvent, failureEvent, command)
   proc successCallback(fd: AsyncFD): bool =
     future.complete(^flowVar)