archive_lib: fix download urls for gs artifacts

BUG=None
TEST=tryjob

Change-Id: I34daad8f48129d15e7a56893ba03921c27cfcef3
Previous-Reviewed-on: https://chromium-review.googlesource.com/344940
(cherry picked from commit 9b7d3ae027a1ea47000ccf101179258d4adf3bb0)
Reviewed-on: https://chromium-review.googlesource.com/345460
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
4 files changed