CI: Add project prefix env to distinguish gradle dependency cache key

The automatically assigned standard 'BUILDKITE_PIPELINE_SLUG'
environment variable is effectively being used as the project prefix.

For more info see: https://github.com/Automattic/
a8c-ci-toolkit-buildkite-plugin/pull/134/commits/
4d3f80b9f113076c9a8698f08d137ed591426150

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This change also brings-in a couple more improvements:
- Measure how long it takes to save/restore the cache.
- Measure the (un)compressed cache size during save/restore.

.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#3.7.1"
+export CI_TOOLKIT="automattic/a8c-ci-toolkit#275fb27c9bcd4d0e9d9a77ca2be733755b83759e"
 export TEST_COLLECTOR="test-collector#v1.10.1"