Use head ref when checking out

[tomuben]

Background

In #143 we fixed the rebuild trigger by checking out the source branch manually. However it would be easier to use the ref parameter for the checkout actions, see https://github.com/actions/checkout?tab=readme-ov-file#checkout-pull-request-head-commit-instead-of-merge-commit

Acceptance Criteria

Use https://github.com/actions/checkout?tab=readme-ov-file#checkout-pull-request-head-commit-instead-of-merge-commit in the actions/checkout workflow instead checking out the source branch manually.