Add ccache to kernel builds (local + CI)

[bleggett]

Helps a bit with long kernel builds

Local results for a single host kernel 6.18.16 build, with docker prunes in between to bust OCI cache:

# Docker prune no ccache build
time sh hack/build/docker-build.sh only-latest-lts
123.58s user 68.49s system 12% cpu 25:14.28 total

# Docker prune ccache build
time sh hack/build/docker-build.sh only-latest-lts
119.09s user 67.52s system 24% cpu 12:44.90 total

~25 mins down to ~13 mins

Improves, but does not close #139

[azenla]

This is great work!