build:project:create does not always autodetect WordPress environment

[jazzsequence]

Running terminus build:project:create with pantheon-systems/wordpress-composer did not autodetect the WordPress environment and defaulted the framework to Drupal8.

This was happening because the wp-config.php file was not set up yet, so when autodetectUpstreamAtDir was checking for the existence of that file, it found nothing and gave up.