Publish linux arm64 binary for Bazel built at commits (#2022)

Fixes https://github.com/bazelbuild/continuous-integration/issues/1402
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 53e5c82..e10dd82 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -463,7 +463,7 @@
     "ubuntu2004_arm64": {
         "name": "Ubuntu 20.04 LTS ARM64",
         "emoji-name": ":ubuntu: Ubuntu 20.04 LTS ARM64",
-        "publish_binary": [],
+        "publish_binary": ["linux_arm64"],
         "docker-image": f"gcr.io/{DOCKER_REGISTRY_PREFIX}/ubuntu2004",
         "python": "python3.8",
         "queue": "arm64",