Add guarded build artifact cache wrapper

[yoshifuminakamura]

Summary

• add a common build cache wrapper for cross build jobs without requiring app build.sh changes
• verify cache hits against build input hashes, current source provenance, and container hashes when present
• require explicit host-build env keys before restoring/storing non-container host caches
• add shell regression coverage and CI/docs updates

Tests

• bash -n scripts/build_with_cache.sh scripts/tests/test_build_cache.sh scripts/matrix_generate.sh
• shellcheck -S error scripts/build_with_cache.sh scripts/tests/test_build_cache.sh scripts/matrix_generate.sh
• bash scripts/tests/test_build_cache.sh
• bash scripts/matrix_generate.sh code=genesis system=RIKYU
• .venv/bin/python -m pytest result_server/tests
• .venv/bin/python scripts/tests/check_text_integrity.py
• git diff --check
• full Result Server Tests shell regression locally