ci: add aarch64 (arm64) Linux release build Build the aarch64-unknown-linux-gnu target natively on GitHub's free ubuntu-24.04-arm runner. The existing gtk4-rs container is amd64-only, so aarch64 runs without a container and installs GTK4/libadwaita via apt. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>