[gitlab-ci] refactor to use except:variables instead of bash if

.gitlab-ci.yml

@@ -20,6 +20,9 @@ check-manifest:
   script: tox -e check-manifest
 
 trigger-other-pipelines:
+  except:
+    variables:
+      - $TRIGGERED_FROM_OTHER_PROJECT
   stage: after-tests
   script:
-  - "if [ -z \"${TRIGGERED_FROM_OTHER_PROJECT}\" ]; then curl -X POST -F \"variables[TRIGGERED_FROM_OTHER_PROJECT]=true\" -F token=$CUBICWEB_PIPELINE_TRIGGER_TOKEN -F ref=branch/default -F \"variables[TRIGGERED_FROM_OTHER_PROJECT]=true\" https://forge.extranet.logilab.fr/api/v4/projects/385/trigger/pipeline; fi"
+  - curl -X POST -F "variables[TRIGGERED_FROM_OTHER_PROJECT]=true" -F token=$CUBICWEB_PIPELINE_TRIGGER_TOKEN -F ref=branch/default https://forge.extranet.logilab.fr/api/v4/projects/385/trigger/pipeline