ci: restore intra-run .turbo cache fan-out to cut CI compute

[y-lakhdar]

Summary

Restores the intra-run .turbo cache fan-out (build once → share as one artifact) that #8146 removed, layered on top of Turbo remote caching, to cut CI compute. Spike — draft until measured and reviewed.

What it does

• After ci: replace artifact Turbo cache with native remote caching #8146, every CI job re-ran turbo build in setup and restored the whole graph from the remote cache (~60s/job; the Run unit tests job showed 57 cached / 62 tasks in ~61s). With ~70 jobs that added roughly +57 job-min per full merge_group run (total ~248 → ~291 job-min), and CI got marginally slower rather than faster.
• The build job now publishes its .turbo once as a per-run artifact (save-cache: 'true'), and needs: build consumers restore it (default load-cache: 'true') for local cache hits instead of each re-fetching ~57 task outputs from the remote cache.
• Restore is best-effort (continue-on-error): on a miss, turbo falls back to the remote cache. turbo.json remoteCache + TURBO_TOKEN are unchanged, so cross-run / cross-branch / local-dev reuse is preserved.
• pr-report, build-windows (different runner OS → different artifact), and build-missing opt out via load-cache: 'false'.

How to evaluate

Compare median total job-minutes of merge_group runs against the current baseline (~291 job-min), targeting a return to ≲250. Also spot-check a consumer job's setup step duration (was 119s on Run unit tests, ~60s of it remote turbo restore).

[changeset-bot]

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[svcsnykcoveo]

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[y-lakhdar]

• wait a couple days to have a better picture of the performances before and after the turbo remote caching implementation