Centralize test timings to fix shard divergence in CI (#90672)
## Summary
For context we discovered jobs were not reliably always running in
https://github.com/vercel/next.js/pull/90668 as addapters + turbopack
group ran more tests than just turbopack group.
- Each sharded test sub-job independently fetched test timings from KV
via a turbo task. Because turbo cache keys can vary across job groups
and KV can return slightly different data at different times, the
sharding algorithm produced different test-to-shard assignments across
groups — causing some tests to be missing entirely and others to run in
multiple shards.
- Adds a `--require-timings` flag to `run-tests.js` that fails loudly if
`test-timings.json` can't be loaded from disk, preventing silent
fallback to KV or round-robin.
- Adds a `testTimingsArtifact` input to `build_reusable.yml` that
downloads a pre-built timings artifact instead of fetching via turbo.
When unset (default), existing behavior is preserved.
- In both `test_e2e_deploy_release.yml` and `build_and_test.yml`,
timings are now fetched once in a setup job, uploaded as a GitHub
artifact, and downloaded by all sub-jobs — guaranteeing identical shard
assignments across all job groups.
## Test plan
- [ ] Verify `build_and_test.yml` jobs pick up the shared `test-timings`
artifact
- [ ] Verify `test_e2e_deploy_release.yml` deploy jobs pick up the
shared `test-timings` artifact
- [ ] Verify workflows that don't pass `testTimingsArtifact` (e.g.
`integration_tests_reusable.yml`, `pull_request_stats.yml`) still use
the turbo fallback path unchanged
