verify_cot fails to validate pull requests on repos that are forks

While testing some bitrisescript changes on staging-firefox-ios, @isabelrios discovered a CoT failure:
https://firefox-ci-tc.services.mozilla.com/tasks/C4i1LZAQS1iLhBbATtJBBw/runs/0/logs/public/logs/chain_of_trust.log

This is a json-e rendering error:

jsone.shared.InterpreterError: InterpreterError at template.tasks[0].repoUrl: infix: . expects objects

It happens because the .taskcluster.yml evaluates event.pull_request.head.repo.html_url (which is standard in all our .taskcluster.yml files), but the Github context that verfiy_cot resolves has:

pull_request:
    head:
        repo: None

I believe this is happening because staging-firefox-ios is a fork of firefox-ios, so it means we hit this condition:
https://github.com/mozilla-releng/scriptworker/blob/cd0aac0510f69386b28a375656b5cc070dc2fb7f/src/scriptworker/cot/verify.py#L1179

This causes us to use mozilla-mobile/firefox-ios as the head repository instead, and my theory is this causes the pull_request.head.repo to be None as the pull request in question doesn't exist over there.