CI tests are running on the wrong branch when the branch is in this repo

[dead-claudia]

Correctly failing run: https://github.com/MithrilJS/mithril.js/actions/runs/16890764018

Run that should've failed like above: https://github.com/MithrilJS/mithril.js/actions/runs/16890688985

[dead-claudia]

This came about from my need to drop 43c4229 to fix the new tests, when #3037's checks should've just failed from the start.