fix: remove placeholder duration from cache metadata (#6756)
### Description
Big oops, noticed that we're saving a placeholder task duration during
cache save.
### Testing Instructions
Inspected cache metadata and it no longer has suspiciously round
numbers:
```
[0 olszewski@chriss-mbp] /tmp/turbo-repo-cache-repro $ bat node_modules/.cache/turbo/d46e88b66977f7f5-meta.json
───────┬─────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ File: node_modules/.cache/turbo/d46e88b66977f7f5-meta.json
───────┼─────────────────────────────────────────────────────────────────────────────────────────────────────────────────
1 │ {"hash":"d46e88b66977f7f5","duration":2292}
```
Closes TURBO-1879
Co-authored-by: Chris Olszewski <Chris Olszewski>
