portable/check-battery: add DBUS env to fix notifications

modules/portable/default.nix

@@ -3,7 +3,7 @@
   services.cron = {
     enable = true;
     systemCronJobs = [
-      "*/5 * * * * root ${pkgs.check-battery}/bin/check-battery"
+      "*/5 * * * * b12f ${pkgs.check-battery}/bin/check-battery"
     ];
   };
 }

pkgs/check-battery.nix

@@ -7,6 +7,7 @@ fi
 
 capacity=$(cat /sys/class/power_supply/BAT0/capacity)
 if [ $capacity -lt 20 ]; then
+  export DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/$(id -u)/bus
   ${libnotify}/bin/notify-send -u critical "Battery ''${capacity}%"
 fi
 ''