communi: fix forbidden build references

pkgs/applications/networking/irc/communi/default.nix

···
       --replace "/usr/bin" "$out/bin"
   '';
 
-  postFixup = ''
+  preFixup = ''
     patchelf --set-rpath "$out/lib:$(patchelf --print-rpath $out/bin/.communi-wrapped)" $out/bin/.communi-wrapped
   '';