test: Really use pnpm on deploy tests (#95360)
We already set `package.json#packageManager`
<img width="539" height="388" alt="Bildschirmfoto 2026-07-01 um 16 26
30"
src="https://github.com/user-attachments/assets/8f6f3e8f-60a6-484d-ac57-f8dc261f8b85"
/>
But the Vercel deployment tests still used npm to install dependencies
because you need to set `ENABLE_EXPERIMENTAL_COREPACK` so that
`packageManager` applies.
pnpm should be hopefully faster, and also improves consistency across
the test environments
Full deploy test run:
~~https://github.com/vercel/next.js/actions/runs/28528683809~~
~~https://github.com/vercel/next.js/actions/runs/28531873334~~
~~https://github.com/vercel/next.js/actions/runs/28542029071~~
~~https://github.com/vercel/next.js/actions/runs/29311003094~~
~~https://github.com/vercel/next.js/actions/runs/32731722428~~
~~https://github.com/vercel/next.js/actions/runs/37841713568~~
https://github.com/vercel/next.js/actions/runs/37900216932
