Print task config for downstream jobs (#1556)

Currently the downstream pipelines execute hundreds of jobs, which makes
it hard to find the correct task config for a specific job. Moreover,
our CI rewrites or skips some tasks, thus adding another layer of
complexity.

This change should it make easier to debug problems with individual
downstream jobs.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 54004df..f76f53d 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1204,8 +1204,12 @@
     if build_only and test_only:
         raise BuildkiteException("build_only and test_only cannot be true at the same time")
 
-    if use_bazel_at_commit and use_but:
-        raise BuildkiteException("use_bazel_at_commit cannot be set when use_but is true")
+    if use_but:
+        if use_bazel_at_commit:
+            raise BuildkiteException("use_bazel_at_commit cannot be set when use_but is true")
+
+        print_collapsed_group(":printer: Printing task config for downstream job...")
+        eprint(json.dumps(task_config))
 
     tmpdir = tempfile.mkdtemp()
     sc_process = None