[ci] Use short-lived access tokens for preview-build uploads instead of static token (#97256)
Replaces the static `PREVIEW_BUILDS_BLOB_READ_WRITE_TOKEN` secret with a
short-lived access token to upload to Vercel Blob in exchange for a
GitHub OIDC (endpoint side:
https://github.com/vercel/vercel-packages/pull/98):
- the job mints GitHub Actions OIDC tokens for the
`https://vercel-packages.vercel.app` audience itself (the workflow gains
`id-token: write`) and re-mints shortly before expiry since a token
lives about five minutes while a package batch can take longer
- `upload-preview-tarballs.js` exchanges the token at vercel-packages
for scoped client-upload tokens via POST and uploads the tarball bytes
directly to Blob storage with `@vercel/blob/client`'s `put`, so the blob
read-write token never leaves vercel-packages
- the `preview-builds` environment is dropped from the job: it only
existed to selectively expose the static token and restrict uploads to
canary, and the canary binding is now enforced by the `job_workflow_ref`
match at vercel-packages.
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Co-authored-by: Claude Code (kimi-k3[1m]) <noreply@anthropic.com>