De-dupe build_and_deploy run for PRs (#78792)
This aims to avoid running the `build_and_deploy` workflow twice when
pushed to the main Next.js repo and also opened as a PR.
Follow-up to https://github.com/vercel/next.js/pull/78737
Validated against:
https://github.com/vercel/next.js/actions/runs/14804173232/job/41569421246?pr=78795
