tests: Fix immutableAssetToken extraction (#90668)
Fixes this deploy test failure:
https://github.com/vercel/next.js/actions/runs/22499483165/attempts/2
The output might be:
```
Building: └ ○ /nested
Building: ○ (Static) prerendered as static content
Building: ƒ (Dynamic) server-rendered on demand
Building: > @ post-build /vercel/path0
Building: > node -e 'console.log("BUILD" + "_ID: " + fs.readFileSync(".next/BUILD_ID") + "\nDEPLOYMENT" + "_ID: " + process.env.NEXT_DEPLOYMENT_ID + "\nIMMUTABLE_ASSET" + "_TOKEN: " + process.env.VERCEL_IMMUTABLE_ASSET_TOKEN)'
Building: BUILD_ID: build-TfctsWXpff2fKS
Building: DEPLOYMENT_ID: dpl_5Qad8NbhCPWn7QRf5TiCePi58vVi
Building: IMMUTABLE_ASSET_TOKEN: undefined
Building: Build Completed in /vercel/output [15s]
Building: Deploying outputs...
Building: Deployment completed
```
Co-authored-by: JJ Kasper <jj@jjsweb.site>
