avahi: set AVAHI_SERVICE_DIR to well-known location
avahi: set AVAHI_SERVICE_DIR to well-known location

Avahi now uses `/etc/avahi/services` instead of its
store path to look for files with service definitions.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs