Return `Collection` from `Ci::Build` and similar classes

app/models/ci/build.rb

app/models/ci/processable.rb

app/models/concerns/ci/contextable.rb

app/presenters/ci/build_runner_presenter.rb

ee/spec/models/ci/build_spec.rb

ee/spec/services/ci/create_pipeline_service/needs_spec.rb

lib/api/entities/job_request/response.rb

lib/expand_variables.rb

lib/gitlab/ci/build/context/build.rb

lib/gitlab/ci/build/context/global.rb

lib/gitlab/ci/pipeline/expression/statement.rb

lib/gitlab/ci/variables/collection.rb

spec/lib/expand_variables_spec.rb

spec/lib/gitlab/ci/build/context/build_spec.rb

spec/lib/gitlab/ci/build/context/global_spec.rb

spec/lib/gitlab/ci/build/policy/variables_spec.rb

spec/lib/gitlab/ci/build/rules/rule_spec.rb

spec/lib/gitlab/ci/build/rules_spec.rb

spec/lib/gitlab/ci/pipeline/expression/statement_spec.rb

spec/lib/gitlab/ci/variables/collection_spec.rb