test: use arrow function

In `test/parallel/test-child-process-env.js`, callbacks use
anonymous closure functions. It is safe to replace them with arrow
functions since these callbacks don't contain references to `this`,
`super` or `arguments`. This results in shorter functions.

PR-URL: https://github.com/nodejs/node/pull/24482


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>