Fix prod deployment: Nix liked to be looking in the wrong folder

Closes #143.
Thanks to http://nixos.wiki/wiki/Nix_Language:_Tips_%26_Tricks for
inspiration.

morph/grid/production/grid.nix

@@ -62,8 +62,8 @@ let
 
   defineStorageNode = name: { vpnIP, stateVersion }:
   let
-    nodecfg = import "${./.}/${name}-config.nix";
-    hardware ="${./.}/${name}-hardware.nix";
+    nodecfg = import (./. + "/${name}-config.nix");
+    hardware = import (./. + "/${name}-hardware.nix");
   in {
     imports = [
       # Get some of the very lowest-level system configuration for this