chore: workflows trigger only on PR events, and not on push

[patriciareinoso]

Description

This change makes the CI trigger only on PR events, and not on push. This will prevent duplicate runs of CI jobs. It will also prevent the scan-rock failing when run on push.

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.