ci: change triggers to prevent dual runs on prs

[clintonsteiner]

• runs on pushes to master or prs to master
• will prevent action from running for the push in pr and pull request into master

[webknjaz]

Personally, I'd recommend disallowing topic branches upstream via branch rulesets and forcing them into forks. This would solve said problem while still allowing branches in forks to be tested on pushes.

[jdavid]

Tests and checks are fast, about 2 minutes. The slow actions (wheels, s390x tests) are only run on push to the master branch. I don't see this to be much of a problem, do I miss something?

[webknjaz]

@jdavid this also only affects PRs from upstream, not forks. But in general, it's a good organizational decision to separate topic branches to they don't collect dust upstream.