speed up execute_async again

When hh-clilib moved to hsl-experimental IO\ , we lost the async
wrappers around arbitrary resources.

Made a mistake when implementing with builtins.

Back to 5 min build

src/__Private/execute_async.php

@@ -27,6 +27,7 @@
 
   list($stdin, $stdout, $stderr) = $pipes;
   \fclose($stdin);
+  \stream_set_blocking($stdout, false);
 
   $exit_code = -2;
   $output = '';