process: use public readableFlowing property

PR-URL: https://github.com/nodejs/node/pull/29502
Refs: https://github.com/nodejs/node/issues/445


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>