Implement --skip-prepare

[CyrilBrulebois]

When patching fails, a few things need to happen:

1. fix the patching, obviously;
2. create appropriate changelog lines accordingly;
3. adjust state.yaml to clean the steps for the failed version;
4. retry, without the preparation (mostly merging, adding a changelog entry), as that was done, not undone, and quite probably augmented with changelog lines documenting the changes.

One way to do the last one is to patch frido/actions.py, setting 'prepare': []. But if that's forgotten, or if --only-one is not passed, we might try building another version without the relevant actions for that step, which would be really needed!

Maybe this could work:

• add a --skip-prepare option;
• if present, it must be accompanied with (--build and) --only-one;
• if present, it overrides the prepare actions with an empty list.