build: add paths-ignore for build-tarball workflow

In GitHub Actions, don't run build-tarball if the only changed files are
docs.

PR-URL: https://github.com/nodejs/node/pull/40109


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>