Fix getLoginURL() for Backdrop

Because of https://github.com/backdrop/backdrop-issues/issues/260, any anonymous visits to /user will redirect to /user/login. Currently CRM_Utils_System_Backdrop::getLoginURL() creates a link to /user?destination... and this URL will automatically try to redirect. And because of Backdrop/Drupal allowing the ?destination parameter to override other redirects, it will automatically go to that destination instead of staying at /user or even /user/login

There's probably an easy fix by just changing getLoginURL to point to /user/login and no immediate redirect will happen.