CI: Enable `experimental.isolatedDevBuild` for `test-integration` (#84558)
Enabling `experimental.isolatedDevBuild` required many changes to the
current workflow, so we will incrementally roll out to the tests.
Enabling on test-integration instead of test-experimental-integration
because `-experimental` CIs are filtered via
`experimental-tests-manifest.json` and they don't cover all tests. We
want to enable this feature by default so we should ensure this
incremental rollout is covered on all test cases.
1. ~~test-experimental-dev
([link](https://github.com/vercel/next.js/pull/84099))~~
2. test-dev ([link](https://github.com/vercel/next.js/pull/84562))
2. test-prod ([link](https://github.com/vercel/next.js/pull/84556))
3. test-integration (here)
4. test-unit
5. Enable by default, remove the flag, and update the rest
x-ref: https://github.com/vercel/next.js/pull/84043