Get rid of our custom CI/CD resumption support

[FooBarWidget]

Github Actions now supports partial reruns. We can now get rid of our custom CI/CD resumption system.

This means that we don't have to store artifacts on Cloud Storage anymore.

One special problem that arises if we stop storing artifacts on Cloud Storage, is that different workflows need to access each other's artifacts. We can possibly implement this by using Github's CI Artifact Download API.

[FooBarWidget]

Actually we still need it in order to make speeding up the CI work.