Set moonset background blur to match the Arch user config

modules/desktop.nix

@@ -88,8 +88,10 @@ in
     "moonlock/moonlock.toml".text = ''
       background_path = "${wallpaper}"
     '';
+    # blur matches the value set in ~/.config/moonset/moonset.toml on Arch.
     "moonset/moonset.toml".text = ''
       background_path = "${wallpaper}"
+      background_blur = 20.0
     '';
 
     # Configs outside /etc/xdg/ in the Arch package as well.