Pin mpv to nixpkgs stable

The unstable version is showing a blue screen instead of the video.

modules/home-manager/desktop/media/mpv.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 with lib;
 
@@ -11,6 +11,7 @@ in
   config = mkIf cfg.enable {
     programs.mpv = {
       enable = true;
+      package = pkgs.stable.mpv;
 
       # https://github.com/mpv-player/mpv/blob/master/etc/input.conf
       bindings = {