Add cachix and enable by default

system/cachix.nix

@@ -0,0 +1,23 @@
+{ pkgs, lib, config, ... }:
+let
+  cfg = config.nixfiles.cachix;
+in
+{
+  options.nixfiles.cachix.enable = lib.mkOption {
+    description = "Whether to enable the Cachix derivation cache";
+    type = lib.types.bool;
+    default = true;
+    example = false;
+  };
+  config = lib.mkIf cfg.enable {
+    nix.settings = {
+      substituters = [
+        "https://hyprland.cachix.org"
+      ];
+
+      trusted-public-keys = [
+        "hyprland.cachix.org-1:a7pgxzMz7+chwVL3/pzj6jIBMioiJM7ypFP8PwtkuGc="
+      ];
+    };
+  };
+}

system/default.nix

@@ -11,6 +11,7 @@ in
     ./programs
     ./sessions
     ./testing
+    ./cachix.nix
   ];
   config = {};
   options.nixfiles = {