Fix sketchybar disk usage showing incorrect percentage

Monitor /System/Volumes/Data instead of / since root is a read-only
APFS snapshot with minimal usage. Also fix inverted formula that was
calculating 100-used instead of just using the capacity value directly.

home/roles/aerospace/default.nix

@@ -632,8 +632,9 @@ in
       text = ''
         #!/bin/bash
 
-        # df $5 can show free% on some APFS setups; calculate used% explicitly
-        DISK_USAGE=$(df -H / | grep -v Filesystem | awk '{gsub(/%/,"",$5); printf "%.0f%%", 100-$5}')
+        # Monitor /System/Volumes/Data which contains user data on APFS
+        # The root / is a read-only snapshot with minimal usage
+        DISK_USAGE=$(df -H /System/Volumes/Data | grep -v Filesystem | awk '{print $5}')
 
         ${pkgs.sketchybar}/bin/sketchybar --set $NAME label="$DISK_USAGE"
       '';