[Chore] Run CI checks on 'pull_request'

[rvem]

Description

Problem: We want to be able to run CI checks on PRs from external forks.
However, this is only possible with 'on: pull_request', while currently
CI is triggered 'on: push'

Solution: Change CI triggering condition to 'on: pull_request'.

Related issue(s)

Fixed #

✅ Checklist for your Pull Request

Related changes (conditional)

• Tests

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from
    silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock

Stylistic guide (mandatory)

• My commits comply with the policy used in Serokell.
• My code complies with the style guide.

✓ Release Checklist

• I updated the version number in package.yaml.
• (After merging) I created a new entry in the releases page,
  with a summary of all user-facing changes.
  • I made sure a tag was created using the format vX.Y