Fix code coverage display for source files in Jenkins UI

As it is not possible to upload cobertura reports produced by Jenkins as a GitLab pipeline artifact (to visualize coverage report directly in the GitLab merge request UI), we have to rely on those available in Jenkins UI until then.

Currently, the painting and display of coverage for Python source files in Jenkins UI is broken. The main issue is a bug in the Code Coverage API Jenkins plugin. I submitted a fix in that pull request, not sure it will be merged but I have updated the plugin in our Jenkins instance with it.

Nevertheless, we also need to update our Jenkins pipelines to ensure proper code coverage display on source files but also to display coverage focused on the changes from a merge request.