Grid configs: clean up config imports

This is a small clean-up from this Slack thread that surfaced in !258 (closed).

Somewhat related to #27 (closed).

morph/grid/local/grid.nix

 let
-  pkgs = import <nixpkgs> { };
-
   gridlib = import ../../lib;
-  grid-config = pkgs.lib.trivial.importJSON ./config.json;
+  grid-config = builtins.fromJSON (builtins.readFile ./config.json);
 
   ssh-users = let
     ssh-users-file = ./public-keys/users.nix;