waybar: Use myxer as a graphical audio mixer

overlays/default.nix

@@ -16,6 +16,7 @@ in {
   });
   nur = import ./nur.nix pkgs;
   unstable = importWrapper ./unstable.nix;
+  stable = importWrapper ./stable.nix;
   custom = import ./custom.nix pkgs;
   simple = import ./simple.nix;
   surface = import ./surface.nix pkgs;

overlays/stable.nix

@@ -0,0 +1,21 @@
+# NOTE: pkgs is unused but there for consistency with nur.nix
+{
+  lib, ...
+}: final: prev:
+
+let
+  stableTarball =
+    fetchTarball
+      https://github.com/NixOS/nixpkgs/archive/nixos-21.05.tar.gz;
+in {
+  stable = import stableTarball {
+    # TODO
+    config = {
+      allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
+        "firmwareLinuxNonfree" "microcodeAmd" "microcodeIntel"
+        "steam" "steam-original" "steam-runtime"
+        "discord"
+      ];
+    };
+  };
+}