Buildkite CI (artifacts): Refactor such that there is a distinct step end when the build finishes

[bkase]

This way later jobs can depend on this part of the artifact build

[O1ahmad]

May make sense to convert Artifact.dhall into a helper Command (like OpamInit or DockerArtifact) to be imported and depended on by other jobs (e.g. integration framework tests). These jobs could then just depend on the command helper's step key after inclusion.

[O1ahmad]

Also as @lk86 pointed out during a discussion artifact builds can already be depended on using the key defined in the job (job end should signal proper readiness of the artifact to consume) and with the provision that Artifact.dhall always runs with any change (which it should now). Only downside here is that the artifact steps are needed (build et all) on every change though this could be avoided with short-circuiting logic.

[O1ahmad]

Dependency keys:

• Debian package upload: _Artifacts-artifacts-build (name: Artifact, key: artifacts-build)
• Docker artifact build/push: _DockerArtifact-docker-artifact (name: DockerArtifact, key: docker-artifact)