Do not run the main CI for MRs

We do not want to run the main CI if the fork happens to use the
main branch, too.

.gitlab-ci.yml

@@ -10,6 +10,8 @@ stages:
   - make html inbox-html inbox-xml pdf xeplist refs xml
   - bash tools/ci-prune-build.sh
   rules:
+  - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    when: never
   - if: '$CI_COMMIT_REF_NAME =~ /^main$/'
     when: always
   - when: never
@@ -37,6 +39,8 @@ stages:
   - 'docker push "$IMAGE_REF"'
   - 'docker push "$LATEST_REF"'
   rules:
+  - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    when: never
   - if: '$CI_COMMIT_REF_NAME =~ /^main$/'
     when: on_success
   - when: never
@@ -52,6 +56,8 @@ stages:
     - state/
     key: attic-state
   rules:
+  - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    when: never
   - if: '$CI_COMMIT_REF_NAME =~ /^main$/'
     when: on_success
   - when: never
@@ -67,6 +73,8 @@ stages:
     - state/
     key: announce-state
   rules:
+  - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    when: never
   - if: '$CI_COMMIT_REF_NAME =~ /^main$/'
     when: on_success
   - when: never