apu: add support for nix-command and flakes, enable gc

hosts/apu/configuration.nix

@@ -116,6 +116,19 @@ in
   networking.hostName = "apu";
   networking.useNetworkd = true;
 
+  nix = {
+    settings = {
+      experimental-features = [ "nix-command" "flakes" ];
+      auto-optimise-store = true;
+    };
+
+    gc = {
+      automatic = true;
+      dates = "daily";
+      options = "--delete-older-than 7d";
+    };
+  };
+
   systemd.network = {
     enable = true;
     config.networkConfig.IPv6Forwarding = true;