add a second settings_location.php

When using command-line tools such as the ones in <civiroot>/bin/csv, we need the settings_location.php to be present in the vendor/civicrm/civicrm-core folder.

There's also some circumstantial evidence that not having this prevents files in the extern folder from working - e.g. working click-through links on CiviMail. I'm not certain about that, since I'm not personally having this problem, but chatter in Mattermost suggests that this is the main difference between my installations and others that have that issue.