Suppress PHP 8 deprecation warning in arcanist_init_script

Summary:
As of PHP 8, the XML entity loader is disabled by default and the
libxml_disable_entity_loader function is deprecated. Thus suppress the
deprecation warning for now; we could skip the function call, but this
is safer.

Test Plan: Used to create this revision with PHP 8 on macOS

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21497