Installing CiviCRM incorrectly defines CIVICRM_UF_BASEURL on Pantheon

When installing CiviCRM on Pantheon, instead of using the base url such as https://www.MYWEBSITE.org it is writing the path to the installer https://www.MYWEBSITE.org/profiles/civicrm_starterkit/modules/civicrm/install/index.php/ to CIVICRM_UF_BASEURL which is incorrect.

This seems to be because in civicrm_cms_base() $_SERVER['SCRIPT_FILENAME'] has a double forward slash "//" and $_SERVER['SCRIPT_NAME'] has only single "/". Then it fails to strip the extraneous bits from the URL. It's replacing double back slashes but not forward slash. So I think we can just clean up the URL a bit more.