ci: speed up build with BuildKit cache + apt cache + GHA tarball cache

[Bennethxyz]

Speed up CI builds (addresses #259):

• Persist BuildKit caches between runs for both agnos and meta-builder images using type=gha cache-from/cache-to in build_system.sh.
• Add apt cache mounts in Dockerfile.agnos to avoid repeated package index downloads and speed up apt installs in multiple stages.
• Cache the Ubuntu base tarball in the workflow via actions/cache to avoid re-downloading on each run.

This should materially reduce build times on subsequent runs while keeping the build reproducible and safe. No behavior changes to the produced images.

Notes:

• Pre-commit shows flake8 error in scripts/diff_dmesg.py unrelated to these changes; not touched here.
• Buildx "--check" steps remain, now reusing cache.