CI: Do not trigger "push" event on pull requests

[exoego]

Purpose

To make CI faster.

Overview

On pull requests, GitHub Actions are executed both on push and pull_request event at this moment.
This is redundant because same CI job is executed twice.
See #94 as example

After this PR, push event is triggered only when main and master branch (when PR is merged).

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary