[test] Authenticate deploy tests via VERCEL_TOKEN instead of shared auth.json (#99846)
Deploy tests now authenticate the Vercel CLI via the `VERCEL_TOKEN` env
var instead of writing a shared `$HOME/.vercel/auth.json` and passing
`--global-config` to every CLI invocation.
This reduces the chance of a read/write race on the shared auth.json.
I've never seen this with our current concurrency limits (2) but we saw
it immediately when bumping to 8
(https://github.com/vercel/next.js/actions/runs/37704518620/job/113083223389#step:35:386)
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Co-authored-by: Claude Code <noreply@anthropic.com>