ffcast: remove unnecessary DESTDIR=$out

This avoids files being installed to $out/nix/store

pkgs/tools/X11/ffcast/default.nix

···
   configureFlags = [ "--disable-xrectsel" ];
 
   postBuild = ''
-    make DESTDIR="$out" install
+    make install
   '';
 
   meta = with stdenv.lib; {