Fix NFSv4 test

http://hydra.nixos.org/build/25349071

nixos/modules/services/network-filesystems/nfsd.nix

···
 
     environment.systemPackages = [ pkgs.nfs-utils ];
 
-    environment.etc = singleton
-      { source = exports;
-        target = "exports";
-      };
+    environment.etc.exports.source = exports;
 
     boot.kernelModules = [ "nfsd" ];
 

nixos/tests/nfs.nix

···
     { config, pkgs, ... }:
     { fileSystems = pkgs.lib.mkVMOverride
         [ { mountPoint = "/data";
-            device = "server:${if version == 4 then "/" else "/data"}";
+            device = "server:/data";
             fsType = "nfs";
             options = "vers=${toString version}";
           }