zsh: turn fpath into a set

Forcing fpath to contain unique values increases startup speed by
eliminating extra work of processing duplicated folders.
In addition, it increases startup time when zsh is enabled in both
system and home configuration due to having the same fpath value
between different compinit calls.
Fixes https://github.com/rycee/home-manager/issues/108.

modules/programs/zsh.nix

@@ -236,6 +236,7 @@ in
       };
 
       home.file."${relToDotDir ".zshenv"}".text = ''
+        typeset -U fpath
         ${envVarsStr}
       '';