This brings back the ability to e.g. configure sane-airscan with
`environment.etc."sane.d/airscan.conf".text = ...`.
(AFAICT, sane-airscan loads all config files it finds, so it'll first
load the one from the nixos hardware.sane.* configuration, then the user
specified one in /etc/sane.d/airscan.conf.)
Fixes: 4fbec87a5b ("nixos/sane: point env vars to /etc for quick reload")
Fixes https://github.com/NixOS/nixpkgs/issues/207262
