[ci] Wire preview builds variables consistently (#99323)
`build_reusable.yml` now defines `NEXT_TEST_PREVIEW_BUILDS_BASE_URL`
next to `PREVIEW_BUILDS_ACCESS`, so every test job inherits both via
`process.env` instead of relying on per-call-site threading. The same
class of gap on next-15-5 broke private-preview deploy tests in the
mirror with an E401 ([failing
run](https://github.com/vercel/next-js-mirror/actions/runs/36334147525/job/108664261165)).
## test plan
- [x] [test_e2e_deploy_release
run](https://github.com/vercel/next.js/actions/runs/36347194442)
Co-authored-by: Claude Code (kimi-k3[1m]) <noreply@anthropic.com>
