The sink-task is just how we make our dynamic task graph work with GitHub's requirement that we name a specific task as 'blocking' for PRs. (Sounds like I should document that clearer in the checks view!).
The actual failure will be the other failing Taskcluster checks. In your case, looks like that's wpt-firefox-nightly-stability. Clicking through to the details, payment-request/allowpayment/no-attribute-same-origin-bc-containers.https.html flaked. Given that all you did (I think?) was rename a variable + move the file, I'd consider this existing flake and am happy to admin-merge the PR if you would like :)
@stephenmcgruer, thanks for clarifying - that makes sense. I didn't realize they were dependent on each other. Would appreciate the merge of #25560, as we've updated the HTML spec and Payment Request Spec already.