ci(ik-llama-cpp,turboquant): add BuildKit ccache mount to compile steps

Mirror the ccache mount added to Dockerfile.llama-cpp in 9228e5b4 for
the other two llama.cpp-derived backends. Same shape, distinct mount
ids so each backend's cache is independent:

  ik-llama-cpp-ccache-${TARGETARCH}-${BUILD_TYPE}
  turboquant-ccache-${TARGETARCH}-${BUILD_TYPE}

ik_llama.cpp is a different upstream fork; no source overlap with
llama-cpp, separate cache makes sense.

turboquant is a llama.cpp fork that reuses backend/cpp/llama-cpp
source via a thin wrapper Makefile — most TUs would in principle hit
llama-cpp's ccache too. Keeping them separate for now to avoid one
fork's regressions poisoning the other; revisit sharing after we have
hit-rate numbers.

Same registry-export behavior as llama-cpp: the cache mount rides on
backend_build.yml's existing cache-to: type=registry,mode=max.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>