zsh: prefer Nix completions these from Zsh package

Zsh ships some rudimentary completions for programs where upstream also
ships their own completions (e.g., curl). So as not to shadow those
completions, we need to prepend to the fpath instead of appending.

NixOS/nixpkgs@8dad5a2

modules/programs/zsh/default.nix

@@ -137,7 +137,7 @@ in
 
       # Tell zsh how to find installed completions
       for p in ''${(z)NIX_PROFILES}; do
-        fpath+=($p/share/zsh/site-functions $p/share/zsh/$ZSH_VERSION/functions $p/share/zsh/vendor-completions)
+        fpath=($p/share/zsh/site-functions $p/share/zsh/$ZSH_VERSION/functions $p/share/zsh/vendor-completions $fpath)
       done
 
       ${cfg.shellInit}