CI: Update a8c-ci-toolkit to pr-sha commit (fix dependency cache)

FYI: This change is done for testing purposes and until the below
'A8C CI Toolkit' #138 PR gets merged to 'trunk'. When that's
done, the 'a8c-ci-toolkit' will be updated to '3.9.1' instead.

A8C CI Toolkit PR: [Dependency Cache] Fix Dependency Cache #138
- Automattic/a8c-ci-toolkit-buildkite-plugin#138

.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#3.9.0"
+export CI_TOOLKIT="automattic/a8c-ci-toolkit#9a12d43b5df24ad661ee565cdff77513dd44216c"
 export TEST_COLLECTOR="test-collector#v1.10.1"