feat: add helix master cache

modules/capabilities/helix.nix

@@ -1,5 +1,14 @@
 { inputs, ... }:
 {
+  flake.modules.nixos.helix = {
+    nix.settings = {
+      extra-substituters = [ "https://helix.cachix.org" ];
+      extra-trusted-public-keys = [
+        "helix.cachix.org-1:ejp9KQpR1FBI2onstMQ34yogDm4OgU2ru6lIwPvuCVs="
+      ];
+    };
+  };
+
   flake.modules.homeManager.helix =
     { pkgs, ... }:
     let