ci: build a snap for linux releases

[Makakashan]

Summary

• package sonora as a strict-confinement snap for amd64 and arm64
• route alsa through a bundled pulseaudio plugin and a triplet-free plugin path so audio works under confinement
• take the vulkan driver from the gpu-2404 provider rather than staging one
• build, inspect and attach a snap to every release, publishing to the snap store only when store credentials are set
• document installing the snap beside the appimage and the flatpak