Teach CoT verify about github-pull-request-untrusted `tasks_for`

[ahal]

This is a new value that's possible after:
https://phabricator.services.mozilla.com/D157493

Might be as simple as adding a second condition to this if statement:

scriptworker/src/scriptworker/cot/verify.py

Line 1315 in c711a7b

elif tasks_for == "github-pull-request":

[escapewindow]

Plausible, yes :) We may also want to adjust valid_tasks_for.

[escapewindow]

And is_pull_request.