build: do not lint on non-PR Travis builds

Do not run any linting at all when `TRAVIS_PULL_REQUEST` is `false`.
This would otherwise break Travis CI for `master` and release branches.

Refs: https://github.com/nodejs/node/pull/24030

PR-URL: https://github.com/nodejs/node/pull/24076


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>