CI/Scripts: small changes to [dune.sh]

Relates to #8110

What

1. Enables dune cache stats for pipelines triggered via api or scheduled pipelines via CI_PIPELINE_SOURCE instead of TZ_SCHEDULE.
2. Hit rate as a float between 0 and 1 with full precision instead of a percentage with two decimal figures.
3. Prints a note when dune cache stats is enabled.

Why

1. Using CI_PIPELINE_SOURCE seems more robust than using TZ_SCHEDULE. NB: we currently have some pipelines triggered via api (cf. ./ci/run_pipeline/api.sh).
2. More flexible and precise than a percentage rate. Probably more standard as well: the raw hit ratio is stored and then the user choses to format it as a percentage in, say, a (Datadog) dashboard.
3. Easier to trace what is happening (is dune.sh called or not, what is the value of DUNE_CACHE_INFO).

How

Simple bash changes.

Manually testing the MR

Check test pipelines.

• schedule_extended_test (CI_PIPELINE_SOURCE=schedule): https://gitlab.com/tezos/tezos/-/pipelines/2217137120
  • oc.build_x86_64-released: dune.sh called with cache stats activated: https://gitlab.com/tezos/tezos/-/jobs/12450721567#L67
• before_merging (CI_PIPELINE_SOURCE=merge_request_event): https://gitlab.com/tezos/tezos/-/pipelines/2217173832
  • oc.build_x86_64-released: dune.sh called with cache stats not-activated: https://gitlab.com/tezos/tezos/-/jobs/12450910041#L67

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR