test: Fix immutable static asset deployment tests for real (#95600)
https://github.com/vercel/next.js/pull/95550 wasn't enough:
-
https://github.com/vercel/next.js/actions/runs/28907225463/job/85769093204
-
https://github.com/vercel/next.js/actions/runs/28907225463/job/85769093252
Because `VERCEL_IMMUTABLE_STATIC_FILES_ENABLED` might be set, but it
still only works when using adapters, so it broke in the non-adapter
deploy test jobs.
Also prepare for when we switch the default of `NEXT_ENABLE_ADAPTER` by
setting it explicitly to `0` for when we want to test the old CLI
builder.