ci: Fix duplicate CI workflow runs on pull requests

[hrj]

Fixes the issue where every commit triggers the CI workflow twice.

By restricting the push event trigger in .github/workflows/ci.yml to the master branch only, PR branches will now only trigger the pull_request event and avoid triggering a duplicate run for the push event.

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PR created automatically by Jules for task 18274624062406751999 started by @hrj

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