It's likely near the softener, but no guarantees. See if the builder or your local building official has an as-built plan they could give you a copy of. That's a good thing to have anyway.
Remember, to be delivering unsoftened water, that line has to originate upstream from the softener. Softeners are usually installed near the point of entry of the water line into the house, so if you poke around between the softener and the POE, you might find what you're looking for. Unfortunately, maybe, since the hose bibs are also on raw water (a good idea, mostly), the upstream tap you'll find probably feeds the hose bibs, and the sink is tapped off that line way downstream. Is there a hose bib on the outside wall near the sink? It may be hard to find the source for the sink. You are probably looking at running a new water line to the sink from the most convenient source of softened water. Any cold water fixture is an eligible candidate, including the toilets, probably. In Phoenix, I'd guess your plumbing is under the slab, but it's also legal and practical to run new piping on the outside walls.Time to get creative.