nixos/zsh: prefer added completions over completions shipped with Zsh

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.

Fixes #197502

nixos/modules/programs/zsh/zsh.nix

@@ -184,7 +184,7 @@ 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
 
         # Setup custom shell init stuff.