Fix sync_bcr_content function to use rsync (#2390)

- Avoid uploading the same files again => should speed up the mirroring
a lot
diff --git a/buildkite/bazel-central-registry/bcr_postsubmit.py b/buildkite/bazel-central-registry/bcr_postsubmit.py
index dd852b9..6a21778 100755
--- a/buildkite/bazel-central-registry/bcr_postsubmit.py
+++ b/buildkite/bazel-central-registry/bcr_postsubmit.py
@@ -153,10 +153,11 @@
 def sync_bcr_content():
     print_expanded_group(":gcloud: Sync BCR content")
     subprocess.check_output(
-        ["gsutil", "-h", "Cache-Control:no-cache", "cp", "./bazel_registry.json", BCR_BUCKET]
+        ["gsutil", "-h", "Cache-Control:no-cache", "rsync", "-c", "./bazel_registry.json", BCR_BUCKET]
     )
     subprocess.check_output(
-        ["gsutil", "-h", "Cache-Control:no-cache", "-m", "rsync", "-r", "./modules", BCR_BUCKET + "modules"]
+        # -c Use checksum to compare files
+        ["gsutil", "-h", "Cache-Control:no-cache", "-m", "rsync", "-c", "-r", "./modules", BCR_BUCKET + "modules"]
     )