Optimize GitHub Actions usage

We currently use between about 45-115 hours of GitHub Actions per day (https://github.com/apache/pekko/actions/metrics/usage, https://infra-reports.apache.org/#ghactions), and some jobs that are ran as part of PR validation can take a long time to run (https://github.com/apache/pekko/actions/metrics/performance).

We should mainly optimize for contributor happiness, so it'd be good if we could see if we can make some of those jobs faster. Also the aggregate amount of consumed resources seems high, if we can improve that as well that would be nice.