Change workflow: into a re-usable reference that can be extended
Problem/Motivation
The workflow: definition is hard-coded in includes/include.drupalci.workflows.yml. Every one of these conditions is a positive test for when a pipeline should run. If a project wants to add another condition to this list, then currently the only to do this is duplicate the entire workflow: defintion and add the new rule. This is tedious, but also duplication means any fixes in gitlab_templates will not be available in the copy.
The example which prompted this is #3503337: Plan for initial codebase but also there are other cases, in other projects.
Proposed resolution
In the above file, convert workflow: to a reusable .default-workflow: &default-workflow then define workflow: to reference this.
Other projects can then define something like (untested) :
workflow:
rules:
- !reference [ .default-workflow ]
- if: $CI_COMMIT_BRANCH == "some-specific-branch"