nixos/syncthing: Add receiveencrypted folder type
nixos/syncthing: Add receiveencrypted folder type

This folder type is available in syncthing, but could not be set in
NixOS.
See <https://docs.syncthing.net/users/untrusted.html> for reference.

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