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

FYI: This change is done for testing purposes and until the below
'A8C CI Toolkit' #135 PR gets merged to 'trunk'. When that's
done, the 'a8c-ci-toolkit' will be updated to '3.9.0' instead.

A8C CI Toolkit PR: [Dependency Updates] [Dependency Cache]
Dependency Cache on CI per Project [without GRADLE_RO_DEP_CACHE] #135
- Automattic/a8c-ci-toolkit-buildkite-plugin#135

.buildkite/shared-pipeline-vars

@@ -3,6 +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#3.7.1"
+export CI_TOOLKIT="automattic/a8c-ci-toolkit#1dda7bd51d20fbb82d5435dc2cbee3b08f5b5153"
 export TEST_COLLECTOR="test-collector#v1.10.1"
-