lib: fix regular expression to detect `/` and `\`

Fixes #40305

PR-URL: https://github.com/nodejs/node/pull/40325


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>