Fixes issue with bogus secret bytes being included when running
`led get-build`. The server would return "<REDACTED>".encode("base64"),
and led would pass this along to the task, which would fail to decode
it as a proto.

R=nodir@chromium.org, vadimsh@chromium.org

Change-Id: Ie8b33a2dc1f05ddb163465d72ea461f9e6833363
Reviewed-on: https://chromium-review.googlesource.com/c/1340504
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>