Verify build cache artifact integrity

[yoshifuminakamura]

Summary

• record artifact tree SHA-256 in the build cache manifest
• verify cached and restored artifacts plus source_info.env before accepting cache hits
• rebuild and refresh cache when cached payload integrity checks fail

Tests

• bash scripts/tests/test_build_cache.sh
• bash scripts/tests/test_build_environment_snapshot.sh
• bash -n scripts/build_with_cache.sh scripts/tests/test_build_cache.sh
• shellcheck -S error scripts/build_with_cache.sh scripts/build_tool_wrappers/bk_build_tool_wrapper.sh scripts/collect_environment_snapshot.sh scripts/tests/test_build_cache.sh
• /home/nakamura/fugakunext/venv/bin/python -m pytest result_server/tests/test_environment_snapshot_ci_scripts.py