ci: prevent to run twice

[drupol]

Related to:

• GitHub Actions has overlapping triggers which causes tests to run twice when pushing to an open PR open-telemetry/opentelemetry-python#1370
• https://stackoverflow.com/questions/70145104/github-actions-running-twice-on-merge-of-pr

[jtojnar]

I sometimes also use CI when pushing to a branch to avoid local builds before opening a PR as described here open-telemetry/opentelemetry-python#1370 (comment) But for this project, I think most of the changes are sufficiently small so I probably open PRs straight away.

Also, in other projects in the past CI would not run on my PR before someone approved it but it would still run in my fork’s branches. This allowed me to fix possible issues before maintainer got around to allowing the CI on the PR. Again, not sure how relevant it is here.

I guess we can try it out.

[drupol]

Thanks!