Ah too bad, I was just about to point that out. Well great that you figured it out on your own!
By the way I think you should consider calling b.step("name", "description") instead of manually allocating it. Then your name and description will be shown in the help menu, and it’s ensured that you don’t accidentally mess with some internal assumptions.
As for the question why cleanup isn’t called: None of your build steps depend on it. So either you have to call it from the build command zig build cleanup_temp_assets or you add a dependency:
exe.step.dependOn(cleanup_step);
cleanup_step.dependOn(build_step);