[CI] Fix on-merge pipeline syntax (elastic#200129)

## Summary

use soft_fail instead of continue_on_failure, as this setting doesn't
exist on command steps
(https://buildkite.com/docs/pipelines/command-step)

.buildkite/pipelines/on_merge.yml

@@ -502,7 +502,7 @@ steps:
       machineType: n2-standard-2
       preemptible: true
     timeout_in_minutes: 60
-    continue_on_failure: true
+    soft_fail: true
     retry:
       automatic:
         - exit_status: '-1'