Backport fixes from: https://github.com/Teriks/dgenerate/tree/flux_newpipes
Fix optional arguments: ``--sdxl-refiner-inference-steps``, ``--sdxl-refiner-guidance-scales``, and ``--sdxl-refiner-guidance-rescales`` not being considered when calculating the number of generation steps displayed to the user.
Minor fix for example runner script, inform user when running from an environment where dgenerate is not accessible as an importable package. In this case, dgenerate will run in a subprocess by invoking the command in a shell if it is available.