CI: Add a pipeline for beta release tag

Context

This merge request add to the CI a pipeline specific to beta release tag.

These tags will be used to publish beta version of major releases. Unlike the release candidate they are not supposed to be stable, thus we do not release opam packages.

Manually testing the MR

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