- Fix Pi kernel build: disable includeDefaultModules in initrd.
NixOS all-hardware.nix includes dw-hdmi which doesn't exist in
the RPi4 kernel 6.12, causing module-shrunk to fail.
- Fix CI: SD image build now depends on build-hosts instead of
flake-check, so it won't run if the Pi build fails.
- Apply same fix to raspberry-pi template.