Generate deterministic pipeline name when it's not given

[plaguss]

Description

When generating a name for the pipeline automatically we were doing it with a random generator, which prevents the cache from working. This changes the behaviour to be deterministic, using the signature of the pipeline (before running, but after closing the context manager so the steps are taken into account even if not all the information is available at that point).

Note: Additionally, a test was failing because it wasn't refreshing the cache folder between tests.

Closes #875