Declare missing class properties in ArcanistDownloadWorkflow

Summary:
rARCbd9769ba92df63d0429f74ec7fb8b00a4989f28b added and declared the three private properties $id, $saveAs, $show.
However rARC21e80a635d798c5be2c6e5c272497b3170c1e079 removed their declarations while still initializing these properties in didParseArguments().

Test Plan: Run PHPStan static code analysis; grep the code in this class.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25713