chore: add our substituter config to flake.nix

Author: jfroche

flake.nix

@@ -1,6 +1,9 @@
 {
   description = "Prototype tooling for deploying PostgreSQL";
-
+  nixConfig = {
+    extra-substituters = [ "https://nix-postgres-artifacts.s3.amazonaws.com" ];
+    extra-trusted-public-keys = [ "nix-postgres-artifacts:dGZlQOvKcNEjvT7QEAJbcV6b6uk7VF/hWMjhYleiaLI=%" ];
+  };
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
     flake-utils.url = "github:numtide/flake-utils";