ci: Use dedicated runners for the build jobs

Context

The change is meant to speed up the build jobs by leveraging the dune cache on the runners.

The runners are configured to use the dune cache, which is a directory mounted on the docker container during the build time. Note that the executing jobs on the dedicated runners is controller by the dedicated tag and is only allocated for the build related jobs.

Relevant issue: #3618 (closed)

Manually testing the MR

Besides the output of this MR, uou could try pushing a branch to a repo where a gitlab-runner instance with the dedicated flag is registered.

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