home: direnv: update to new option name

And enable the flakes option.

I don't actually use this on my own projects, but it could be useful for
other projects that I contribute to.

home/direnv.nix

@@ -9,7 +9,11 @@ in
 
   config.programs.direnv = lib.mkIf cfg.enable {
     enable = true;
+    nix-direnv = {
       # A better `use_nix`
-    enableNixDirenvIntegration = true;
+      enable = true;
+      # And `use_flake`
+      enableFlakes = true;
+    };
   };
 }