CI: Update a8c-ci-toolkit to pr-sha commit (test download dependencies)

FYI: This change is done for testing purposes only, and in order to
verify that the newly added 'init.gradle.kts' script is working as
expected.

A8C CI Toolkit PR: [Dependency Cache] Test Custom Init Gradle Task
to Download Dependencies #142
- Automattic/a8c-ci-toolkit-buildkite-plugin#142

.buildkite/shared-pipeline-vars

@@ -3,5 +3,5 @@
  # This file is `source`'d before calling `buildkite-agent pipeline upload`, and can be used
  # to set up some variables that will be interpolated in the `.yml` pipeline before uploading it.
 
-export CI_TOOLKIT="automattic/a8c-ci-toolkit#3.9.1"
+export CI_TOOLKIT="automattic/a8c-ci-toolkit#4aed409fb7535d16c4e08794997fad3773389a9b"
 export TEST_COLLECTOR="test-collector#v1.10.1"