Cheat our way through arc version on Windows for the moment

Summary: Fixes T8291. See PHI52. This is papering over the real issue (T8298) but it's a 10-second patch so just improve things slightly for now.

Test Plan: Ran arc version locally; patch confirmed on a Windows system by an affected user.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8291

Differential Revision: https://secure.phabricator.com/D18518