fix(build): fix passing CMAKE_<LANG>_FLAGS from ExternalProject_Add

tools/test_apps/build_system/external_project/main/CMakeLists.txt

@@ -0,0 +1,2 @@
+idf_component_register(SRCS "test_main.c"
+                       REQUIRES ext_component)

tools/test_apps/build_system/external_project/main/test_main.c

@@ -0,0 +1,12 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+
+#include <stdio.h>
+
+void app_main(void)
+{
+    printf("external_project test OK\n");
+}