fix : missing onnxruntime dependency for coverage job

required for MR aidge_onnx!75

.gitlab/ci/ubuntu_python.gitlab-ci.yml

@@ -76,7 +76,7 @@ coverage:ubuntu_python:
     - source venv/bin/activate
     - which python
     - python -m pip list
-    - python -m pip install numpy coverage requests unittest-xml-reporting pytest
+    - python -m pip install numpy coverage requests unittest-xml-reporting pytest onnxruntime
   script:
     - cd ${CI_PROJECT_NAME}
     # Retrieve the installation path of the module, since it is installed with pip.