shell.nix: don't override NIX_CONF_DIR

shell.nix

@@ -10,15 +10,10 @@ in pkgs.mkShell {
 
   shellHook = ''
     mkdir -p secrets
+    PATH=${
+      pkgs.writeShellScriptBin "nix" ''
+        ${pkgs.nixFlakes}/bin/nix --option experimental-features "nix-command flakes ca-references" $@
+      ''
+    }/bin:$PATH
   '';
-
-  NIX_CONF_DIR = let
-    current = pkgs.lib.optionalString (builtins.pathExists /etc/nix/nix.conf)
-      (builtins.readFile /etc/nix/nix.conf);
-
-    nixConf = pkgs.writeTextDir "opt/nix.conf" ''
-      ${current}
-      experimental-features = nix-command flakes ca-references
-    '';
-  in "${nixConf}/opt";
 }