build: only lint version numbers for pull requests

Fixes: https://github.com/nodejs/node/issues/39850

PR-URL: https://github.com/nodejs/node/pull/40027


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>