Don't try to guess the location of the NixOS config file

The NixOS config need not be $NIXOS_CONFIG, it can also be set through
-I nixos-config=... or not exist in a separate file at all (e.g. in a
NixOps deployment).

Issue #212.

modules/services/misc/nix-daemon.nix

@@ -34,8 +34,9 @@ let
       pkgs.runCommand "nix.conf" {extraOptions = cfg.extraOptions; } ''
         extraPaths=$(for i in $(cat ${binshDeps}); do if test -d $i; then echo $i; fi; done)
         cat > $out <<END
-        # WARNING: this file is generated from the nix.* entries in
-        # ${maybeEnv "NIXOS_CONFIG" "/etc/nixos/configuration.nix"}
+        # WARNING: this file is generated from the nix.* options in
+        # your NixOS configuration, typically
+        # /etc/nixos/configuration.nix.  Do not edit it!
         build-users-group = nixbld
         build-max-jobs = ${toString (cfg.maxJobs)}
         build-use-chroot = ${if cfg.useChroot then "true" else "false"}