ci: run a single GitLab CI pipeline

Commit to fix the current behavior of GitLab that triggers two CI
pipelines for the same commit.

For example, both the pipelines
[114510](https://gitlab.desy.de/asapo/asapo/-/pipelines/114510) and
[114511](https://gitlab.desy.de/asapo/asapo/-/pipelines/114511) ran on
the same commit 56d16797. See
asapo/asapo!214

.gitlab-ci.yml

+---
+workflow:
+  rules:
+    - if: $CI_COMMIT_BRANCH
+
 include:
   - local: deploy/build_env/services-linux/gitlab-ci-build-docker.yml
   - local: deploy/build_env/services-windows/gitlab-ci-build-docker.yml