The package sets for an image can depend on the blueprint, and
by the same logic there is no reason it should not be able to
depend on the image options.
This is so far a non-functional change, but makes a follow-up
commit simpler (though still without actually depending on
the image options to compute the package sets).
The value doesn't represent the worker name, just the top-level cache
directory for a job. It's useful for separating caches and making the
generation faster, but it's not necessary to return from the function.
