I’m a newbe on NixOS.
I installed abcde (cd ripper) and want to change the output file to .flac.
abcde.conf was on Debian in my home directory, on nix I found it under /nix/store/gpqyflnfx6w7ifswpaxqi9sdj0a7sm7z-abcde-2.9.3/etc/abcde.conf.
You should still be able to configure abcde via the ~/.abcde.conf file on NixOS. The path you found is for the default configuration, which you can use as a starting point for your own conf file if you want. (In other distributions, that file lives in /etc.)