tools: fix mixup with bytes.decode() and str.encode()

We want to read a bytes file and decode the contents as utf-8 so we can
compare against a utf-8 pattern.

PR-URL: https://github.com/nodejs/node/pull/29208


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>