nix: add channel configuration and rework update process

nix/home-manager/programs/zsh.nix

@@ -3,7 +3,7 @@
 , ...
 }:
 
-let 
+let
 in {
   programs.zsh = {
     enable = true;
@@ -16,15 +16,18 @@ in {
 %_%F{%(!.red.green)}$(prompt_char)%f '
       RPROMPT=""
 
-      # Automatic rehash 
+      # Automatic rehash
       zstyle ':completion:*' rehash true
 
       if [ -f $HOME/.shrc.d/sh_aliases ]; then
-        . $HOME/.shrc.d/sh_aliases 
+        . $HOME/.shrc.d/sh_aliases
       fi
 
-      source ${pkgs.homeshick}/homeshick.sh
-      fpath=(${pkgs.homeshick}/completions $fpath)
+      ${if builtins.hasAttr "homeshick" pkgs then ''
+        source ${pkgs.homeshick}/homeshick.sh
+        fpath=(${pkgs.homeshick}/completions $fpath)
+      '' else ''
+      ''}
 
       # Disable intercepting of ctrl-s and ctrl-q as flow control.
       stty stop ''' -ixoff -ixon