Use latest CI toolkit version

Author: mokagio

.buildkite/pipeline.yml

@@ -7,7 +7,7 @@ env:
 # Nodes with values to reuse in the pipeline.
 common_params:
   plugins: &common_plugins
-    - automattic/a8c-ci-toolkit#3.1.0
+    - automattic/a8c-ci-toolkit#3.5.1
 
 # This is the default pipeline – it will build and test the app
 steps: