Ensure NEXT_TEST_MODE is set for production run (#63474)

## What?

Missed this env var when adding the additional config.

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Author: timneutkens

.github/workflows/build_and_test.yml

@@ -195,7 +195,7 @@ jobs:
     uses: ./.github/workflows/build_reusable.yml
     with:
       nodeVersion: 18.17.0
-      afterBuild: RUST_BACKTRACE=0 NEXT_EXTERNAL_TESTS_FILTERS="$(pwd)/test/turbopack-build-tests-manifest.json" TURBOPACK=1 TURBOPACK_BUILD=1 node run-tests.js --timings -g ${{ matrix.group }} -c ${TEST_CONCURRENCY} --type production
+      afterBuild: RUST_BACKTRACE=0 NEXT_EXTERNAL_TESTS_FILTERS="$(pwd)/test/turbopack-build-tests-manifest.json" TURBOPACK=1 TURBOPACK_BUILD=1 NEXT_TEST_MODE=start node run-tests.js --timings -g ${{ matrix.group }} -c ${TEST_CONCURRENCY} --type production
       stepName: 'test-turbopack-production-${{ matrix.group }}'
     secrets: inherit