Don't run updates during install

Problem/Motivation

Currently, Scheduled Updates run during install and, depending on your configuration, will always print something similar to following message at the end of install:

Updater Publish multiple nodes at a certain time complete. Results: No updates were performed.
Updater Publish single node at certain time complete. Results: No updates were performed.

This is because Scheduled Updates run on cron, and the installer runs cron as part of install_finished(). We've had complaints that this is confusing to people who are installing Lightning - especially on Acquia Cloud where the default for one-click installs is through the UI.

Proposed resolution

I can't think of a situation where you would have a Scheduled Update that you wanted to run during install. In Scheduled Update's hook_cron, we should just bail out if drupal_installation_attempted() is true.

Remaining tasks

Patch forthcoming.