Absolute paths for artifacts and cache don't work in shell runners

As it turns out, we are using absolute paths to declare to-be cached files and artefacts, typically based on the settings for ${CONDA_ROOT}, which is in turn located inside ${CI_PROJECT_DIR}.

While this path is well situated inside the project build directory, the behaviour of the runners differ with respect to the fact the path is absolute:

1. Both docker-based and macos (shell) runners understand that the path is located within the project directory and can find the files to be cached or transported to dependent stages of the build
2. The Linux shell runner does not like the fact the path is set in absolute terms and refuses to cache or zip artefacts from the builds.