[ci] Add flag to keep build-on-ready functionality (elastic#198397)

## Summary
add `build_on_ready=true`

Needed after: elastic/buildkite-pr-bot#84

.buildkite/pull_requests.json

@@ -14,6 +14,7 @@
       "build_on_commit": true,
       "build_on_comment": true,
       "build_drafts": false,
+      "build_on_ready": true,
       "trigger_comment_regex": "^(?:(?:buildkite\\W+)?(?:build|test)\\W+(?:this|it))|^\\/ci$",
       "always_trigger_comment_regex": "^(?:(?:buildkite\\W+)?(?:build|test)\\W+(?:this|it))|^\\/ci$",
       "skip_ci_labels": ["skip-ci"],