CI: split [build_x86_64] into released / un-released

Context

With the latest snapshot, we're hitting the 1GB artifact size limit imposed by GitLab. In this workaround, we split the x86_64 job in two: one that builds the released executable, and another that builds the experimental executables, the dev executables and the extras (tezt & tps evaluation).

The artifact of both jobs is roughly 500 MB. In addition to distributing the artifact size, it should lower pipeline wall time since the two jobs both run faster than old build_x86_64 does.

Manually testing the MR

Trust the CI.

Go check the two new jobs, and verify that the union of their artifacts is indeed the artifacts of the old build_x86_64.

Check the size of the two new jobs. With the API, you can do:

curl --silent --header "PRIVATE-TOKEN: glpat-YOURTOKEN" https://gitlab.com/api/v4/projects/tezos%2Ftezos/jobs/JOB_ID/ | jq '.artifacts_file.size / (1024*1024)'

to get the artifact size of a job in megabytes.

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