Hi @watanave,
Thank you for your patience.
I have spoke with our tooling and website team. This is apparently intended behaviour for open source/public repositories to prevent unauthorized access to your caches (since anyone can open a pull request on your project).
I am going to open a JIRA internally to discuss the possibility/feature request of adding the option for users to disable this precaution. In the meantime, the solution is to start builds on the branch itself, not the pull request.
Please don’t hesitate to follow-up if you have any questions.
Kind regards,
Luna