- Replace nuclear 'docker system prune -af' and 'docker volume prune -af'
  with targeted removal of only the image we just built
- Add clean-gcc/clean-clang Makefile targets that remove specific image
  from both linuxkit cache and docker
- Add BuildKit cache pruning with configurable quota via BUILDX_KEEP_STORAGE_MB
  env var (defaults to 20000 MB) to prevent unbounded disk growth
- Print BuildKit disk usage before pruning for diagnostics
- Use buildctl inside linuxkit-builder container for cache management
- Switch pr-build.yml from pull_request_target to pull_request
- Remove unnecessary Docker Hub login from PR builds

Signed-off-by: Mikhail Malyshev <mike.malyshev@gmail.com>
(cherry picked from commit 04d9224)