report-only-changed-files does not work for forks when using `workflow_run`

[cecheta]

When using workflow_run or workflow_dispatch, the base and head are determined directly from the branches of the PR. However, this does not work for forks, because the head branch is in a different repo.

This can be used by using label instead of ref, as this includes the repo owner, e.g. MishaKav:main