WIP: updated benchmarking script to work with Darwin

[Gregory-Pereira]

Relates to: https://github.com/sigstore/model-transparency/issues/78

Summary

The benchmarking script does not work with Darwin, in such a way that it appears to be suffering under load while it actually just doesn't work in a automated fashion. This PR will seek to update the benchmarking script to work for both Linux and Darwin.

Release Note

Use jq and .json files as results store instead of bash associative arrays

Documentation

No documentation should be impacted by this.