ci: run develop pipelines when source is nightlies

Allow pipelines originating from another pipeline like `nightlies` to
run even if not on the default branch.

.gitlab-ci.yml

@@ -3,6 +3,7 @@ workflow:
     - if: $CI_PIPELINE_SOURCE == 'web'
     - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
     - if: $CI_PIPELINE_SOURCE == 'parent_pipeline'
+    - if: $CI_PIPELINE_SOURCE == 'pipeline'
     - if: $CI_COMMIT_TAG
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH