CI: -j 12 for debian/ubuntu build packages jobs in release pipelines

What

Add DUNE_BUILD_JOBS: -j 12 for debian and ubuntu packages build jobs only in the release pipelines.

Why

These jobs fails with OOM in the release pipeline.

We limit the dune build jobs in this jobs to resolve the OOM, the wall time consequences are acceptable as this is a release pipeline which is not run often.

Manually testing the MR

Check https://gitlab.com/nomadic-labs/tezos/-/pipelines/2082998488, especially oc.build-debian and oc.build-ubuntu jobs.

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