Use a different GCS bucket for storing last green commits (#2040)

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index ce45ac4..106c909 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -3644,14 +3644,14 @@
 
 
 def bazelci_last_green_commit_url(git_repository, pipeline_slug):
-    bucket_name = "bazel-testing-builds" if THIS_IS_TESTING else "bazel-untrusted-builds"
+    bucket_name = "bazel-testing-builds" if THIS_IS_TESTING else "bazel-untrusted-last-green-commits"
     return "gs://{}/last_green_commit/{}/{}".format(
         bucket_name, git_repository[len("https://") :], pipeline_slug
     )
 
 
 def bazelci_last_green_downstream_commit_url():
-    bucket_name = "bazel-testing-builds" if THIS_IS_TESTING else "bazel-untrusted-builds"
+    bucket_name = "bazel-testing-builds" if THIS_IS_TESTING else "bazel-untrusted-last-green-commits"
     return "gs://{}/last_green_commit/downstream_pipeline".format(bucket_name)