use the action cache for passing tests (#93954)
## Summary
Memoize passing tests across attempts of the same workflow run, so a
re-attempt after a timeout or cancellation can skip what already passed.
## What changed
- `run-tests.js` appends each passing test's filename to
`$NEXT_TEST_PASSED_FILE` as it finishes (sync `write` + `fsync`, fd kept
open across calls). On startup it reads the same file and skips any
entries that match.
- `build_reusable.yml` wraps the test step with `actions/cache/restore`
(before) and `actions/cache/save` (after, with `if: always()`). The save
step runs on success, failure, and cancel — which is the whole point: a
cancelled attempt still gets to preserve its progress for retry.
- Cache key is scoped to
`${input_step_key}-${run_id}-attempt${run_attempt}`, with `restore-keys`
falling back to any earlier attempt of the same run.
The previous implementation wrote a single Turbo-remote-cache blob
*after* `Promise.allSettled` resolved — so a cancellation (like [run
26127371192](https://github.com/vercel/next.js/actions/runs/26127371192/job/76845288867))
lost every passed test from that attempt.
This also removes one dependency on the turbo-remote cache (there are
still two more uses of `scripts/turbo-cache.mjs`)
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