build: use Node.js 14 in commit-lint.yml

All of our workflows use Node.js 14.x or 16.x except for commit-lint.yml
which has 12.x hard-coded. Update it to 14.x and change it to using an
environment variable so it is consistent with our other workflows.

PR-URL: https://github.com/nodejs/node/pull/39506


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>