nix: add Max's cache

I trust you, Max (please don't let this be a bad choice...)

modules/system/nix/module.nix

@@ -148,10 +148,12 @@ in {
       # Substituters to pull from.
       substituters = [
         "https://cache.nixos.org"
+        "https://cache.privatevoid.net"
       ];
 
       trusted-public-keys = [
         "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+        "cache.privatevoid.net:SErQ8bvNWANeAvtsOESUwVYr2VJynfuc9JRwlzTTkVg="
       ];
     };
   };