Added job_build_artifacts to required-tests gate (#28206)

ref #28197

`job_build_artifacts` is missing from `job_required_tests.needs`, so
when it fails the gate doesn't see it. Its downstream jobs
(`job_ghost-cli`, `job_build_e2e_image`, `job_e2e_tests`) cascade into
"skipped" via their own `needs:` / `if:` checks, and the gate's
`contains(needs.*.result, 'failure')` check sees only success/skipped —
green merge despite a real release-pipeline failure.

Surfaced concretely in #28197: pack.js broke under pnpm 11, `Build &
Publish Artifacts` went red, the three downstream jobs skipped, gate
passed, the .npmrc-deletion regression landed on main (devcontainer +
production Docker + next release would all have failed).