lib: simplify own keys retrieval

This replacement can save us a function call, two array spreadings,
and an array concatenation.

PR-URL: https://github.com/nodejs/node/pull/24582
Refs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Reflect/ownKeys#Description


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>