Fix hardcoded path in tiny_benchmarks

## Describe the bug
The `tiny_benchmarks` code contained a hardcoded file path to a pickle file, which caused a `FileNotFoundError` when run in environments other than the author's.

## To Reproduce
Run `tiny_benchmarks` on any machine where the hardcoded path does not exist.

## Expected behavior
The code should use relative paths or `pathlib` to locate resources within the package.

## Version info
- OS: mac
- Lighteval version: main (local development)