Parallel benchmarking

[moltar]

Parallelization had to be disabled a while ago, because the final step of the benchmarking run (per Node version) is to commit back the results. This resulted in a race condition, between runs and some would fail to commit, as they had a stale git checkout.

I think we can fix this by running the benchmarks in parallel, but instead of committing the results right away, use GitHub actions artifacts mechanism to store the results in the cache.

Then as the last step, get the cached results at once, and commit them at once.