Add comment to disable_debug_output in cc-test/build.rs

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

dev-tools/cc-test/build.rs

@@ -157,6 +157,8 @@ fn run_action_if_forked() -> bool {
 }
 
 fn disable_debug_output() {
+    // That env would break tests for warning/debug output,
+    // and it is set in the CI, to make debugging CI failure easier.
     std::env::remove_var("CC_ENABLE_DEBUG_OUTPUT");
 }