verify_cot fails to validate pull requests on repos that are forks

[ahal]

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:

scriptworker/src/scriptworker/cot/verify.py

Line 1179 in cd0aac0

if repo_definition["fork"]:

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.

[ahal]

My intuition is that the if repo_definition["fork"] logic is wrong and should be deleted. Whether a repo is a fork or not shouldn't matter, we should only care about where the task is running.

But I'm not super confident about this and would love some more eyes on this.

[ahal]

I can confirm that removing the if repo_definition["fork"] block fixes Isabel's failure.

[bhearsum]

Did #646 resolve this?