Wow, thanks! So for future reference, what I did was:
# Create the local fontconfig directory
$ mkdir -p ~/.config/fontconfig/conf.d/
# Create a file in that directory for the Nix fonts
$ cat << EOF > ~/.config/fontconfig/conf.d/10-nix-fonts.conf
<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
<dir>~/.nix-profile/share/fonts/</dir>
</fontconfig>
EOF
Since these steps are (I presume) identical for any standard Nix setup in non-NixOS systems, I wonder if it would be possible or desirable for the installation of fonts by Nix to either automatically set these up, or at least print instructions to do so manually. Do you have any thoughts on that?