Improve CI speed

Summary

Currently the CI takes to long, we should take steps to reduce the runtime of the pipelines.

Possible fixes

• specifying jobs as interruptible will reduce load on the CI server and improve pipeline times https://docs.gitlab.com/ee/ci/yaml/README.html#interruptible
• the needs: keyword can be used to build a DAG of pipeline jobs, which we could possibly use to improve the runtime https://docs.gitlab.com/ee/ci/yaml/README.html#needs
• fix the caching already reported in #147