Include artifact modes in build cache hash

[yoshifuminakamura]

Summary

• include directory and regular-file mode bits in the build cache artifact tree hash
• add a regression test for mode-only artifact changes
• verify executable artifacts are restored with executable mode after rebuild

Tests

• bash scripts/tests/test_build_cache.sh
• bash -n scripts/build_with_cache.sh scripts/tests/test_build_cache.sh
• shellcheck -S error scripts/build_with_cache.sh scripts/tests/test_build_cache.sh
• git diff --check