fix(turbopack/napi): Flush optional task cache hit statistics upon completion of build (#75122)
```
cd ~/front/apps/next-site
rm -rf .next && NEXT_TURBOPACK_TASK_STATISTICS=~/build-stats.json TURBOPACK=1 TURBOPACK_BUILD=1 TURBOPACK_PERSISTENT_CACHE=0 /bin/time -v pnpm next build --experimental-build-mode=compile
jq 'to_entries | sort_by(.value.cache_hit, -.value.cache_miss) | from_entries' <build-stats.json >build-stats-sorted.json
```
Gives the following `build-stats-sorted.json` file: https://gist.github.com/bgw/cd3bafbbe08a428334ed9bec10885769