Time to upgrade nix on my Macbook. Ran into

---- oh no! --------------------------------------------------------------------
It seems the build user _nixbld15 already exists, but with the UID
'31008'. This script can't really handle that right
now, so I'm going to give up.

during the install. I just ran through the users deleting them to let the installer figure it out afterwards

sudo /usr/bin/dscl . -delete /Users/_nixbld15