prevent signing-event from running in forks

[jku]

If a signer has enabled GitHub Actions in their fork, running tuf-on-ci-sign <event> leads to signing-event workflow running on their fork too: this is confusing so undesirable

• The easy solution is likely if: github.repository_owner == 'upstream-org-name' in the workflow but we can't include that in the template workflow: it has to be done by each project
• Alternatively we could make the action a no-op if the repository is a fork in general... that sounds like it could backfire