nix: set NIX_PATH per device versions at buildtime

Previously all devices would have NIX_PATH set on the common verion file
defindd in *nix/variables/versions.nix*. Now NIX_PATH is populated with
the versions of the respective device.

nix/home-manager/programs/zsh.nix

@@ -1,13 +1,8 @@
-{ pkgs
-, config
-, ...
-}:
+{ pkgs }:
+
+{ ... }:
 
 let
-  channelSources = (import ../../default.nix {
-      versionsPath = ../../variables/versions.nix;
-    }).channelSources;
-
   just-plugin =
     let
       plugin_file = pkgs.writeText "_just" ''
@@ -71,7 +66,7 @@ in {
       # don't cd into directories when executed
       unsetopt AUTO_CD
 
-      source ${channelSources}
+      export NIX_PATH="${pkgs.nixPath}"
 
       # print lines without termination
       setopt PROMPT_CR