ci: avoid spawning 2 pipelines for each commit

When an MR is open, you can have both a normal and a "detached"
pipeline.

They have different properties, but a detached pipeline is
enough for us and identical to the normal one, so we can avoid
the extra load on the CI.

https://docs.gitlab.com/ee/ci/jobs/job_control.html#avoid-duplicate-pipelines

https://docs.gitlab.com/ee/ci/yaml/index.html#switch-between-branch-pipelines-and-merge-request-pipelines

.gitlab-ci.yml

@@ -26,6 +26,16 @@ include:
 
   - local: "ci/images_template.yml"
 
+workflow:
+  rules:
+    - if: $CI_MERGE_REQUEST_IID
+    # don't create a pipeline if its a commit pipeline, on a branch and that branch has
+    # open merge requests (bc we will get a MR build instead)
+    - if: $CI_OPEN_MERGE_REQUESTS
+      when: never
+    - if: $CI_COMMIT_TAG
+    - if: $CI_COMMIT_BRANCH
+
 variables:
   FDO_UPSTREAM_REPO: gstreamer/gstreamer-rs
   RUST_DOCS_FLAGS: "--extern-html-root-url=muldiv=https://docs.rs/muldiv/1.0.0/muldiv/ -Z unstable-options"