stream: avoid writing to writable

A remainder from a previous refactoring.

Refs: https://github.com/nodejs/node/pull/31197

PR-URL: https://github.com/nodejs/node/pull/31805


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>