add gpuBackend/gpuAdapter module options for wgpu device selection under Nix

[DarthPJB]

The NixOS and Home Manager modules now expose two options for controlling wgpu device selection:
programs.ratty = {
gpuBackend = "vulkan"; # "vulkan" | "gl" | "gles" | null
gpuAdapter = "RTX 3060"; # substring match | null
};
gpuBackend — Sets WGPU_BACKEND to force the graphics API. Cases:

• "vulkan" — Explicit Vulkan path. Needed when the default backend auto-detection picks a broken backend, or when testing driver-specific behaviour.
• "gl" / "gles" — OpenGL fallback. Useful on systems where Vulkan is unavailable or broken (headless, VNC, older hardware, driver bugs).
• null — Auto-detect (default). wgpu picks the best available backend.
  gpuAdapter — Sets WGPU_ADAPTER_NAME to select a specific GPU by name substring. Cases:
• Multi-GPU systems (integrated + discrete, or dual discrete) where wgpu picks the wrong adapter.
• Testing against a specific GPU without disabling others in BIOS.
• Offloading rendering to a secondary GPU while keeping the primary for display.
  The NixOS module composes these into a single makeWrapper call alongside the existing --config-file flag. The Home Manager module sets them via home.sessionVariables.

[DarthPJB]

@orhun - I am unsure if this is the best way to pass in an environment variable like this; I am not 'rust-native' - Please feel free to amend the approach or suggest a better methodology.

[orhun]

thank you!