Add support for an optional tempdir_root parameter.

As it is, when ->to() takes a \$scalar , it automatically creates a tempdir in $PWD, which some people don't like too much.

This allows specifying where the tempdir will be created via ->tempdir_root , which defaults to the value File::Spec->rel2abs(Cwd::cwd).

This feature also conveniently works around rt#44924 : https://rt.cpan.org/Ticket/Display.html?id=44924