build: pin doc workflow to Node.js 20

To save time, the `.github/workflows/doc.yml` workflow runs
with a pre-built Node.js. The switch of `lts/*` to Node.js 22 has
broken this workflow for Node.js 20 and 18 due to a mismatch
in globals. Pin the version of Node.js back to 20.

PR-URL: #55755
Fixes: #55754
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: richardlau