ci: rebuilding the benchmarks should check for reproducibility

The point of the `rebuild` CI task is to ensure that we can reliably reproduce the benchmarks included with Sightglass. IIRC, there was a time when that task checked if the benchmark output had changed, e.g., with `git diff --exit-code`. That check is no longer in place but I believe it should be re-added so we can more confidently claim that we have reproducible benchmark builds.