ASV nightly results are hard to find

The benchmarking results of the nightly runs are not presented in the rendered benchmark pages. That's totally understandable, as they're not associated with a merge commit, but just the time of day.

There's an outstanding request to have regressions send notifications, but outside of that, it's still hard to find the comparison between the two nights of data.

I propose adding the output of asv-formatter as a "job summary" on the workflow.

See: https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#adding-a-job-summary