Improve file name of shard distribution (#2051)

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index c0cd71e..135d794 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -2466,7 +2466,8 @@
             s: get_targets_for_shard(sorted_test_targets, s, shard_count)
             for s in range(shard_count)
         }
-        path = os.path.join(tmpdir, "shard_distribution.json")
+        base = f"{os.getenv('BUILDKITE_PIPELINE_SLUG')}_{os.getenv('BUILDKITE_BUILD_NUMBER')}_shards.json"
+        path = os.path.join(tmpdir, base)
         with open(path, mode="w", encoding="utf-8") as fp:
             json.dump(data, fp, indent=2, sort_keys=True)