Fix CI to fetch master on beta channel
This forward-ports a fix from the beta channel (landing in #86413, hopefully) to master so that we don't need to apply it on each round of backports.
This bug also demonstrates that our channel-checking is a bit insufficient -- stable is checked, but beta has some of its own peculiarities currently and isn't checked. But this does not attempt to adjust for that; we likely can't afford to run both beta and stable channels by CI and the current state here seems OK for now.
r? `@pietroalbini`