Attempt to fix permissions for forks

[abelbraaksma]

See: https://github.com/dorny/test-reporter#recommended-setup-for-public-repositories

[abelbraaksma]

It's unclear whether this first needs to be in main to be triggered. Currently, it doesn't trigger the new test-report.yaml at all (unless I deliberately add a syntax error, which does get reported).

EDIT: the workflow file must exist on main before it gets triggered, so let's merge. See: https://github.com/orgs/community/discussions/25288