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