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}

infra/config/lib/ci.star

@@ -110,6 +110,7 @@ def set_defaults(milestone_vars, **kwargs):
         service_account = "chromium-ci-builder@chops-service-accounts.iam.gserviceaccount.com",
         swarming_tags = ["vpython:native-python-wrapper"],
         triggered_by = [milestone_vars.ci_poller],
+        goma_use_luci_auth = True,
     )
     default_values.update(kwargs)
     for k, v in default_values.items():