How? What cloud providers offer it? MacStadium and AWS don't.

I guess you could have a physical MBP in your house and connect it to some bring-your-own-infrastructure CI setup, but most people wouldn't want to do that.

I meant using a physical device indeed.

Cloud providers don't seem too relevant to a discussion of CI for kernel and driver development.

Why not?

How do you imagine that a cloud computing platform designed around running Macs with macOS would work for testing an entirely different OS running on bare metal on hardware that doesn't have a BMC, and usefully catching and logging frequent kernel panics and failed boots?

It's a pretty hard problem to partially automate for setups with an engineer in the room. It doesn't sound at all feasible for an unattended data center setup that's designed to host Xcode for compiling apps under macOS.

GitHub’s self hosted runners are as painless as they can get, and the Mac Mini in my basement is way faster than their hosted offering.

Getting M4 Mac Mini CI might be a while considering the amount they've changed the package. Too tall to go into a 1U, smaller so the rack frames need to be redesigned. Power button now on the bottom so switch actuation need finagling.