add t2 firmware substitutor

hosts/modules/core/t2-firmware.nix

@@ -7,15 +7,15 @@
     inputs.nixos-hardware.nixosModules.apple-t2
   ];
 
-  environment.systemPackages = with pkgs; [
-    # t2-firmware dependencies
-    # REF: https://github.com/t2linux/nixos-t2-iso
-    python3
-    dmg2img
-    # get-apple-firmware
-  ];
+  nix.settings = {
+    trusted-substituters = [
+      "https://t2linux.cachix.org"
+    ];
+    trusted-public-keys = [
+      "t2linux.cachix.org-1:P733c5Gt1qTcxsm+Bae0renWnT8OLs0u9+yfaK2Bejw="
+    ];
+  };
 
-  hardware = {
     apple.touchBar.enable = true;
 
     # REF: https://github.com/NixOS/nixos-hardware/blob/master/apple/t2/default.nix