Detect Ubuntu GDM configuration path

scripts/Install-SguLinuxRustDeskClient.sh

@@ -230,7 +230,11 @@ configure_x11_login_screen() {
 
     case "$display_manager" in
         gdm3.service|gdm.service)
-            if [[ $display_manager == gdm3.service ]]; then
+            # Ubuntu exposes the unit as gdm.service on some releases while
+            # the package still reads /etc/gdm3/custom.conf. Prefer the
+            # distribution-specific directory instead of inferring it only
+            # from the unit name.
+            if [[ -d /etc/gdm3 || -f /etc/gdm3/custom.conf ]]; then
                 configuration_path='/etc/gdm3/custom.conf'
             else
                 configuration_path='/etc/gdm/custom.conf'