nix: enable optimisation of the store

modules/nix/default.nix

@@ -5,10 +5,16 @@
       automatic = true;
       dates = "weekly";
       options = "--delete-older-than 20d";
+      # Catch up on missed runs due to being powered off
+      persistent = true;
     };
-    settings.experimental-features = [
+    settings = {
+      # Deduplicate identical files in the store regularly
+      optimise.automatic = true;
+      experimental-features = [
         "nix-command"
         "flakes"
       ];
     };
+  };
 }