Fix an issue with BuildLogs and web UI during builds

Summary:
Ref T9123. After recent changes, viewing a live build log from the web UI throws a CSRF exception.

Check start ("this object is an active log"), not live ("this log is open somewhere").

Test Plan: Will push / etc.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9123

Differential Revision: https://secure.phabricator.com/D14185