src: reduced substring calls

Reduced the number of substring calls by 1 as it is a linear time
complexity function. Thus having a larger path might lead to decrease in
performance. Also removed unnecessary string allocation happening in the
block.

PR-URL: https://github.com/nodejs/node/pull/34808


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>