Can I ask why this is something you need? It’s probably in the gulp configuration.
Edit
Little bit of digging, if you check the gulpfile it shows you the object where the directory is being stored (I’m on my phone so no line counts for you). I haven’t ever tried, but manually changing the path should work. Just add this is after where the path variable is declared;
path.dist = your/custom/directory/
It seems to be set as default by asset-builder, check out the docs there for more info (though I personally find the docs pretty confusing). This seems to be the relevant part to you.