Fix build output path of the malioc core list (#39629)

impeller/tools/malioc.gni

diff --git a/impeller/tools/malioc.gni b/impeller/tools/malioc.gni
index 17e8456..afecc09 100644
--- a/impeller/tools/malioc.gni
+++ b/impeller/tools/malioc.gni
@@ -14,7 +14,7 @@
 }
 
 if (impeller_malioc_path != "" && impeller_malioc_cores == []) {
-  core_list_file = "$root_gen_dir/mali_core_list.json"
+  core_list_file = "$root_build_dir/mali_core_list.json"
   exec_script("//build/gn_run_binary.py",
               [
                 rebase_path(impeller_malioc_path, root_build_dir),