miku: Use unstable channel

hosts/miku.nix

@@ -47,6 +47,8 @@ in mkHost config {
     multimc-wrapped # Custom package
   ];
   extraOptions = old: {
+    system.autoUpgrade.channel = "https://nixos.org/channels/nixos-unstable";
+
     programs = {
       mpv.primaryScreen = "C27F398";
       adb.enable = true;