gs_cache: fall back to service account when default auth fails BUG=b:179721419 TEST=manually tested on chromeos2-devservertest Change-Id: I1528d4ee7e9e68400a5b0011174e6a9e3053d5f2 Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2690206 Commit-Queue: Congbin Guo <guocb@chromium.org> Tested-by: Congbin Guo <guocb@chromium.org> Auto-Submit: Congbin Guo <guocb@chromium.org> Reviewed-by: Allen Li <ayatane@chromium.org>