Reland "infra/config Use Goma's task service account."

This reverts commit ccb9f3bf.
I noticed I need to update other GCS permission to allow
the task service account to run gclient sync.

Bug: b/166712857
Change-Id: I4e534eb1bb1a026de169b2c0b58e9bd50b354b26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398070Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805166}

infra/config/generated/realms.cfg

@@ -109,7 +109,7 @@ realms {
   name: "goma"
   bindings {
     role: "role/buildbucket.builderServiceAccount"
-    principals: "user:chromium-ci-builder@chops-service-accounts.iam.gserviceaccount.com"
+    principals: "user:goma-release-testing@chops-service-accounts.iam.gserviceaccount.com"
   }
   bindings {
     role: "role/buildbucket.owner"

infra/config/subprojects/goma/goma.star

@@ -32,7 +32,7 @@ defaults.execution_timeout.set(3 * time.hour)
 defaults.os.set(os.LINUX_DEFAULT)
 defaults.pool.set("luci.chromium.ci")
 defaults.service_account.set(
-    "chromium-ci-builder@chops-service-accounts.iam.gserviceaccount.com",
+    "goma-release-testing@chops-service-accounts.iam.gserviceaccount.com",
 )
 defaults.swarming_tags.set(["vpython:native-python-wrapper"])
 defaults.triggered_by.set(["master-gitiles-trigger"])