CI: Setup perf regression alert on PRs

I'd find it interesting to use https://github.com/benchmark-action/github-action-benchmark in CI for pull requests to know that a perf regression had happened, like in their screenshot:

This would supplement Effekt Plots (which runs on `master` periodically), and would be especially helpful for random drive-by contributions.

However, I don't really care too much about the action's GitHub Pages rendering -- that we do better with Effekt Plots.