live generator: don't error out when not in a live session

it's perfectly normal!

mkosi.extra/usr/lib/systemd/system-generators/kde-linux-live-generator

@@ -14,7 +14,7 @@ late_dir=$3
 
 if ! grep "kde-linux.live=1" /proc/cmdline; then
     echo "kde-linux.live=1 not in cmdline"
-    exit 1
+    exit 0
 fi
 
 if [ "$(readlink --canonicalize /dev/disk/by-label/KDELinuxLive)" != "$(readlink --canonicalize /dev/gpt-auto-root)" ]; then