fix check update (#9444)

check_software_update runs in a new thread, won't return directly

Signed-off-by: 21pages <sunboeasy@gmail.com>

flutter/lib/consts.dart

@@ -570,3 +570,5 @@ enum WindowsTarget {
 extension WindowsTargetExt on int {
   WindowsTarget get windowsVersion => getWindowsTarget(this);
 }
+
+const kCheckSoftwareUpdateFinish = 'check_software_update_finish';