Include cache in flake

flake.nix

@@ -1,6 +1,13 @@
 {
   description = "A Nix flake for Conky, including a dev shell";
-
+  nixConfig = {
+    substituters = [
+      "https://conky.cachix.org"
+    ];
+    trusted-public-keys = [
+      "conky.cachix.org-1:4H7kaqUIbxZO5LReWGyzwh3ktfYJb/K7E9au2gssyzM="
+    ];
+  };
   inputs = {
     nixpkgs = {
       url = "github:nixos/nixpkgs/nixos-unstable";