rattler-build's source.path copy does not include submodule contents —
directories like mods/silo/, mods/solver/, mods/gears/, and mods/datums/
end up empty in the work directory, causing cmake --install to fail
when it tries to install files from those paths.
Add a clone_if_empty helper to build.sh that detects empty submodule
directories and shallow-clones them from their remotes before CMake
runs. Add git to build requirements in recipe.yaml.
This fixes the release build (AppImage) failure where cmake --install
failed at the silo addon install step after successfully installing
the silo tree-node icons from src/Mod/Create/resources/.
