PR tests are, in some circumstances, checking out the `main` branch rather than the PR branch

[dead-claudia]

H/T @kfule in MithrilJS/mithril.js#3021 (comment) for bringing this up.

Related workflow run: https://github.com/MithrilJS/mithril.js/actions/runs/14931181879/job/41947598107?pr=3021#step:2:62