zsh: Disable compiling in /usr/local/... as the site fndir

With the default configure flags, zsh compiles in a reference to the
"site function" directory that's outside the store path. Under the
right (wrong) circumstances, other builds using zsh can be affected by
these functions.

pkgs/shells/zsh/default.nix

@@ -26,6 +26,7 @@ stdenv.mkDerivation {
     "--with-tcsetpgrp"
     "--enable-pcre"
     "--enable-zprofile=${placeholder "out"}/etc/zprofile"
+    "--disable-site-fndir"
   ];
 
   # the zsh/zpty module is not available on hydra