Fix bug where script_directory is being overwritten by %dbic_dh_args

script_directory is getting overwritten by contents of %dbic_dh_args when using --target_dir options.

Before this change when using --target_dir my output dir would look like this:
  ls share/
  fixtures  MySQL  _source

This is because the dbic_dh_args hash gets a key named script_directory:

print Dumper \%dbic_dh_args;

$VAR1 = {
          'databases' => [
                           'MySQL'
                         ],
          'script_directory' => '/asc/array1/Projects/dists/Apace-Schema-ACL/share'
        };

so the the script_directory key is instantly overwritten and the migrations dir is never concatenated.