fix(turbopack-bench): Limit `copy_dir` concurrency to avoid running out of file descriptors (#77468)
When running
```
RUST_BACKTRACE=1 cargo test --benches --release -p turbopack-bench 'bench_startup'
```
I was seeing
```
routine failed: Too many open files (os error 24)
```
Fix this by adding a semaphore.