Merge the extract-source job into the build's job script
To follow salsa-ci-team!569, that switches the pipeline to use sbuild and the unshare backend.
This completely removes the provisioning stage. The customization made in the extract-source job is now adapted and handled directly in the build job. After creating the source package with dpkg-buildpackage, sbuild is told to build the binary packages using the .dsc (via BUILD_COMMAND).