add nil, nixfmt and agenix to the flake devshell

flake.nix

@@ -126,6 +126,9 @@
         {
           "${system}".default = pkgs.mkShell {
             packages = [
+              inputs.agenix.packages.x86_64-linux.default
+              pkgs.nil # Nix language server
+              pkgs.nixfmt-rfc-style
               pkgs.terraform-ls
               pkgs.deploy-rs
               pkgs.zsh