perf: refine fetch runtime usage

[MrCroxx]

What's changed and what's your intention?

Please explain IN DETAIL what the changes are in this PR and why they are needed. :D

As title. Use read runtime for fetch, use user runtime for fn.

Checklist

• I have written the necessary rustdoc comments
• I have added the necessary unit tests and integration tests
• I have passed make all (or make fast instead if the old tests are not modified) in my local environment.

Related issues or PRs (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
foyer/src/hybrid/builder.rs	96.49% <ø> (ø)
foyer/src/hybrid/cache.rs	82.60% <100.00%> (+0.53%)	⬆️