add backup cron job

devices/nas.nix

@@ -73,4 +73,11 @@
   networking.useDHCP = lib.mkDefault true;
 
   nixpkgs.hostPlatform = lib.mkDefault "aarch64-linux";
+
+  services.cron = {
+   enable = true;
+   systemCronJobs = [
+     "* 20 * * *     root rsync -Pav -e \"ssh -i /root/.ssh/backup -o ServerAliveInterval=120\" /srv backup@192.168.178.24 >> /root/backup.log"
+   ];
+  };
 }