findutils: updatedb now uses writable database outside of /nix/store by default

updatedb could only be run by providing the --output parameter,
because it would use a path inside the nix store as it's database.
The default for --output is now /var/cache/locatedb (the same
as in the NixOS locate service)

pkgs/tools/misc/findutils/default.nix

@@ -17,6 +17,8 @@ stdenv.mkDerivation rec {
 
   outputs = [ "out" "info" ];
 
+  configureFlags = [ "--localstatedir=/var/cache" ];
+
   crossAttrs = {
     # http://osdir.com/ml/bug-findutils-gnu/2009-08/msg00026.html
     configureFlags = [ "gl_cv_func_wcwidth_works=yes" ];