add basic stat monitoring to backup script

modules/system/backups.nix

@@ -124,11 +124,15 @@ in
               exit 0
             fi
           }
-          time -f "\
+          start_time=$(date +%s)
-          Real time: %E\n\
+          backup
-          User CPU: %U\n\
+          end_time=$(date +%s)
-          Sys CPU: %S\n\
+          exec_time=$((end_time - start_time))
-          Max memory: %M KB" backup
+          cpu_usage=$(top -bn1 | grep "Cpu(s)" | sed "s/.*, *\([0-9.]*\)%* id.*/\1/" | awk '{print 100 - $1"%"}')
+          echo ""
+          echo "backup stats:"
+          echo "exec time: $exec_time"
+          echo "cpu usage: $cpu_usage"
         '';
       };
     };