Continuous kernel build

Rolling kernel build, refreshed on every successful main build.
Consumed by nextbsd-kernel-extensions (kernel-obj-<arch>.tar.gz) and
nextbsd (nextbsd-kernel-<arch>.tar.gz, plus
rpi5-kernel8-arm64.img for the Raspberry Pi 5 family image lanes;
rpi500-kernel8-arm64.img is a transition alias for the same
file).

Build log: https://github.com/nextbsd/nextbsd-kernel/actions/runs/34016502484
Commit: d0fcafc