Default to use a task service account on CI builders.

All builders using lib/ci.star will use the task service account
by default.

All the task service accounts used for accessing Goma have already been
granted.  The CL should not cause crbug.com/1122461.

Bug: crbug.com/1105814
Change-Id: Ibb925c3a7edaf123d9efc4ab1c0a3b112bee451e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380965


Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805628}