Don't check for updates in demo mode

src/Backends/SystemUpdate.vala

@@ -65,6 +65,10 @@ public class SettingsDaemon.Backends.SystemUpdate : Object {
     }
 
     public async void check_for_updates (bool force, bool notify) throws DBusError, IOError {
+        if (Utils.is_running_in_demo_mode ()) {
+            return;
+        }
+
         if (current_state.state != UP_TO_DATE && current_state.state != AVAILABLE && !force) {
             return;
         }

src/Utils/SessionUtils.vala

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2024 elementary, Inc. (https://elementary.io)
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+namespace SettingsDaemon.Utils {
+    public static bool is_running_in_demo_mode () {
+        var proc_cmdline = File.new_for_path ("/proc/cmdline");
+        try {
+            var @is = proc_cmdline.read ();
+            var dis = new DataInputStream (@is);
+
+            var line = dis.read_line ();
+            if ("boot=casper" in line || "boot=live" in line || "rd.live.image" in line) {
+                return true;
+            }
+        } catch (Error e) {
+            critical ("Couldn't detect if running in Demo Mode: %s", e.message);
+        }
+
+        return false;
+    }
+}