Users with 'Administer software updates' permission prevented from running update.php

Problem/Motivation

When attempting to update from Drupal 8.4.5 to 8.5.0, I see the error "In order to run update.php you need to either be logged in as admin or have set $settings['update_free_access'] in your settings.php." when visiting the update.php page.

I have attempted to run these updates as both a user with the "Administrator role", and when logged in as user 1, and I'm still getting the error.

In further testing, this seems to be happening only on my Acquia environments, and not on my local site.

Proposed resolution

* TBD

Remaining tasks

* Find out why this is happening
* Fix it, if necessary

User interface changes

* See https://www.drupal.org/project/drupal/issues/2955673

API changes

* None (probably)

Data model changes

* None (probably)