add cachix config to flake.nix

Author: tennox

flake.nix

@@ -92,4 +92,12 @@
           uboot-rpi-arm64 = pinned.uboot-rpi-arm64;
         } // kernels;
     };
+
+  nixConfig = {
+    extra-substituters = [ "https://nix-community.cachix.org" ];
+    extra-trusted-public-keys = [
+      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+    ];
+  };
+
 }