Harden build workflow against corrupted Lake cache restores

[Copilot]

The build GitHub Actions job was failing during lake build when a restored .lake cache contained a package checkout with an unreadable git HEAD. This change makes the workflow recover from corrupted cached package repos instead of failing immediately, and avoids re-saving broken cache state.

• Cache recovery

  • Add a cleanup step after restoring .lake to prune cached package directories whose git HEAD cannot be resolved.
  • This lets Lake refetch only the broken dependency checkout instead of treating the entire cache as usable.

• Cache write safety

  • Save the Lake cache only on successful builds.
  • This prevents a failed run from persisting corrupted package state back into the shared cache.

• Workflow behavior

  • The change is isolated to .github/workflows/build_book.yml; build inputs and build commands are unchanged.

- name: Remove corrupt cached packages
  run: |
    if [ -d .lake/packages ]; then
      find .lake/packages -mindepth 1 -maxdepth 1 -type d | while read -r package; do
        if ! git -C "$package" rev-parse HEAD >/dev/null 2>&1; then
          rm -rf "$package"
        fi
      done
    fi

- name: Save Lake cache
  if: success()