Upload index generated from different branches into their own subdirectories (#1349)

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 75cf87b..818b734 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1532,7 +1532,8 @@
 
     if index_upload_gcs:
         pipeline = os.getenv("BUILDKITE_PIPELINE_SLUG")
-        destination = KZIPS_BUCKET + pipeline + "/" + final_kzip_name
+        branch = os.getenv("BUILDKITE_BRANCH")
+        destination = KZIPS_BUCKET + pipeline + "/" + branch + "/" + final_kzip_name
         print("Uploading to GCS {}".format(destination))
         execute_command([gsutil_command(), "cp", final_kzip_name, destination])