[ci] Add retry to saved object migration step (#191034)

This is running on a spot instance and should retry if the agent is
lost.

.buildkite/pipelines/on_merge.yml

@@ -589,6 +589,11 @@ steps:
       preemptible: true
     artifact_paths:
       "target/plugin_so_types_snapshot.json"
+    timeout_in_minutes: 30
+    retry:
+      automatic:
+        - exit_status: '-1'
+          limit: 3
 
   - wait: ~
     continue_on_failure: true