Setup builder as cache and activate hydra

modules/nixos/builder/default.nix

@@ -29,10 +29,14 @@ in
   };
 
   config = lib.mkIf cfg.enable {
+    nix.distributedBuilds = true;
+    nix.settings.builders-use-substitutes = true;
+
     nix.buildMachines = [
       {
         hostName = "builder.julian-mutter.de";
         protocol = "ssh";
+        sshUser = "nix";
         systems = [
           "x86_64-linux"
           "aarch64-linux"
@@ -48,8 +52,5 @@ in
         mandatoryFeatures = [ ];
       }
     ];
-    nix.distributedBuilds = true;
-    # optional, useful when the builder has a faster internet connection than yours
-    # nix.extraOptions = "  builders-use-substitutes = true\n";
   };
 }