Optimize GitHub Actions usage

[raboof]

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.

[He-Pin]

Maybe we can stop 1.0.0-nightly build?

[raboof]

Maybe we can stop 1.0.0-nightly build?

I think we should keep that around until we officially EOL the 1.0.x branch - which is something we might want to discuss but perhaps not here and not yet ;). We could configure it to only run when there's actual new commits to that branch, though - that might already help quite significantly.

[pjfanning]

• the 1.0 job could be run weekly or on demand - we probably don't need it nightly since it is not a very active branch
• the main focus should be on cutting down the PR CI time because this makes up most of the hours