child_process: replace var with const/let in internal/child_process.js

PR-URL: https://github.com/nodejs/node/pull/30414


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>