ci: use artifacts for e2e prep so job retries don't fail on cache eviction

[GermanJablo]

Switches the e2e-prep handoff from actions/cache to actions/upload-artifact + actions/download-artifact, so that re-running an individual failed E2E shard works without having to re-run the entire workflow (or push an empty commit to re-trigger CI, which is what we've been doing).

This is also the pattern recommended by GitHub: cache is for dependencies reused across runs, artifacts are for passing data between jobs within a run. e2e-prep falls squarely into the second category.

Why

The Actions cache for this repo is permanently at its 10 GB limit, so the e2e-prep-<sha> entry gets evicted by LRU within a few hours of being saved. When that happens, any retry of a failed shard fails at the "Restore prepared test environment" step with Failed to restore cache entry, and the only recovery path is re-running the full workflow. Artifacts are scoped to the workflow run and not subject to the 10 GB cache budget, so they survive across attempts.

Real example that motivated this: https://github.com/payloadcms/payload/actions/runs/24524398342/job/71809444756

Notes for reviewers

• include-hidden-files: true is required because test/node_modules contains .bin, .pnpm, etc. Missing that flag silently produces a broken artifact.
• Added an explicit "Verify prepared test environment" step with an actionable error, since download-artifact has no fail-on-cache-miss equivalent.
• Left the default 90 day retention. This repo is public, so artifact storage is free and there's no reason to be aggressive.
• Upload/download is slightly slower than cache (zip vs zstd), but the difference is in the order of tens of seconds on a job that takes minutes. Worth it for the reliability win.
• e2e-prep-<sha> cache key is left behind on old runs; it will naturally age out on its own, no cleanup needed.

This does not touch the separate restore-build cache used by .github/actions/setup. That one still has its own polling + full-build fallback, which is orthogonal to this change.

[github-actions]

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[denolfe]

Just some comment cleanup needed, looks good otherwise.