fix: Handle non-directory files in cache folder when performing cleanup (#84930)
https://vercel.slack.com/archives/C046HAU4H7F/p1760563400531829
Repro this by creating a non-directory file in `cache/turbopack`, and then invalidating it:
```
touch .next/dev/cache/turbopack/notadirectory
touch .next/dev/cache/turbopack/__turbo_tasks_invalidated_db
```
Before:
After:
See that the directory was cleaned up.
next.js
fb106d54 - fix: Handle non-directory files in cache folder when performing cleanup (#84930)
