Define SSoT factory method for Ci::Build including definition

The following discussion from !203321 (merged) should be addressed:

• @lma-git started a discussion: (+1 comment)

  Wdyt of the approach here?

  attributes = build_attributes.merge(yaml_variables: yaml_variables, options: options, ...)
  
  # set definition as temp_job_definition.
  build = Ci::Build.fabricate(attributes, for_bulk_insert: true)
  
  # uses build_job_definition and the whole job could be persisted together.
  build = Ci::Build.fabricate(attributes)