nixos: use nix store path for zsh lib instead of mutable worktree path

libDir pointed to ${config.dotfiles-worktree}/dotfiles/lib, which gets
evaluated at build time — so running `just switch` from inside a worktree
would bake that worktree path into /etc/zshenv permanently. If the worktree
is later removed, every zsh startup errors with "no matches found".

zshLibDir was already defined via builtins.path (a stable nix store copy)
but unused. Switch all fpath/autoload/PATH references to it and drop libDir.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nixos/environment.nix

@@ -11,7 +11,6 @@
     if envDotfilesWorktree != ""
     then envDotfilesWorktree
     else "/srv/dotfiles";
-  libDir = "${config.dotfiles-worktree}/dotfiles/lib";
   zshLibDir = builtins.path {
     path = ../dotfiles/lib;
     name = "dotfiles-zsh-lib";
@@ -77,9 +76,9 @@ in
           # handled once by oh-my-zsh below, with this check intentionally skipped.
           ZSH_DISABLE_COMPFIX=true
 
-          fpath=("$HOME/.lib/completions" "${libDir}/completions" $fpath)
-          fpath+="${libDir}/functions"
-          for file in "${libDir}/functions/"*
+          fpath=("$HOME/.lib/completions" "${zshLibDir}/completions" $fpath)
+          fpath+="${zshLibDir}/functions"
+          for file in "${zshLibDir}/functions/"*
           do
               autoload "''${file##*/}"
           done
@@ -184,7 +183,7 @@ in
           export STARSHIP_INSIDE_EMACS="yes"
         '';
         extraInit = ''
-          export PATH="$HOME/.cargo/bin:${libDir}/bin:${libDir}/functions:$PATH";
+          export PATH="$HOME/.cargo/bin:${zshLibDir}/bin:${zshLibDir}/functions:$PATH";
         '';
       };
     };