Default to use a task service account on try builders.

All builders using lib/try.star will use the task service account
by default.  All the task service accounts used for accessing
Goma have already been granted.

Bug: crbug.com/1105814
Change-Id: Ib0bfd2feb6d04453f87801a56c276eae13bfb6a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2408882Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806575}

infra/config/lib/try.star

@@ -121,6 +121,7 @@ def set_defaults(milestone_vars, **kwargs):
         service_account = "chromium-try-builder@chops-service-accounts.iam.gserviceaccount.com",
         swarming_tags = ["vpython:native-python-wrapper"],
         task_template_canary_percentage = 5,
+        goma_use_luci_auth = True,
     )
     default_values.update(kwargs)
     for k, v in default_values.items():