Skip config validation job in downstream pipeline (#1835)

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 9bfcef3..c5d5c83 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -2925,7 +2925,7 @@
             )
         )
 
-    if "validate_config" in configs:
+    if "validate_config" in configs and not is_downstream_pipeline():
         pipeline_steps += create_config_validation_steps(
             git_commit or os.getenv("BUILDKITE_COMMIT")
         )