Save time during build by not optimizing the store

modules/nix-settings.nix

@@ -5,11 +5,13 @@ _: {
       "flakes"
     ];
 
-    auto-optimise-store = true;
-
     trusted-users = [ "@wheel" ];
 
     # RPi builds can be slow due to compiling via binfmt.
     download-buffer-size = 1 * 1024 * 1024 * 1024;
   };
+
+  # Instead of nix.settings.auto-optimise-store, which adds overhead to each
+  # build.
+  nix.optimise.automatic = true;
 }