CI Cache visibility: know which caches are used in each job

What

• Create branch where all caches are enabled in all jobs
  • push-only policy
  • for all daily pipelines (schedule_extended_test + *.daily except opam.daily)
• List on datadog for each cache and job, non-empty caches after the job.

Why

Comparing the results with master, we may learn that in some jobs, enabling a given cache could make sense.