This commit fixes an issue with LND, in which if both
nix-bitcoin.onionServices.lnd.public &
services.lnd.restOnionService.enable were enabled, one would try to
create a file named `lnd` and the other would try to create a directory
named `lnd` with a file named `lnd-rest` inside it. This would obiously
cause an error and fail the LND service.