fix(ci): Trying to fix benchmark-turbopack workflow (#8090)

### Description

Trying to fix the workflow by checking if the directory exists, a team
member should check.


![Screenshot_20240521_182347](https://github.com/vercel/turbo/assets/53424436/a4dab48f-9514-4147-93ee-0775aefef624)

### Testing Instructions

<!--
  Give a quick description of steps to test your changes.
-->

.github/workflows/bench-turbopack.yml

@@ -294,7 +294,11 @@ jobs:
 
  - name: Copy benchmark results
  run: |
- find artifacts -size 0 -delete
+ if [ ! -d artifacts ]; then
+ echo "Artifacts directory not found. Skipping copy."
+ exit 0
+ fi
+ find artifacts -size 0 -delete || true
  mkdir -p data/${{ steps.date.outputs.year }}/${{ steps.date.outputs.month }}/ubuntu-latest-8-core/${{ steps.date.outputs.date }}-${{ github.sha }}/
  mv artifacts/bench_* data/${{ steps.date.outputs.year }}/${{ steps.date.outputs.month }}/ubuntu-latest-8-core/${{ steps.date.outputs.date }}-${{ github.sha }}/