signed-off-by: skip rule when PR-URL trailer is present not only Backport-PR-URL?

The signed-off-by rule currently skips backports identified by a Backport-PR-URL: trailer. This works for already-landed backport commits, but fails for backport PRs at review time because the commit-queue only adds Backport-PR-URL: at land time. even though the contributing doc explicitly exempts backports.

Rule 1 says:
https://github.com/nodejs/node/blob/e15f90559bc62b1e9a8296f1fc8654f5be261baa/doc/contributing/pull-requests.md?plain=1#L207-L208

Rule 2 says:
https://github.com/nodejs/node/blob/e15f90559bc62b1e9a8296f1fc8654f5be261baa/doc/contributing/backporting-to-release-lines.md?plain=1#L77

The lint (in core-validate-commit) says: if no Signed-off-by: AND no Backport-PR-URL: (which is added by the commit-queue), fail.

Suggestion

Treat PR-URL: trailer as the same kind of skip signal as Backport-PR-URL:. Both trailers are added by the commit-queue at land time they're never contributor-authored. Their presence on a commit being validated is a strong heuristic that the commit was either cherry-picked from a previously-landed PR (the dominant case, all backports), or is itself a previously-landed commit having its message re-validated.

core-validate-commit の Signed-off-by ルールから始めて、Backport-PR-URL: trailer の既存の処理を追跡してください。バリデーションテストがあれば確認し、PR-URL: を含むコミットが Signed-off-by なしで受け入れられる一方、それ以外のコミットでは現在の要件が維持されることを確認してください。