Option for custom temporary directory for WSClean instead of /tmp

During the finalize step, WSClean's temporary directory defaults to /tmp. This does not have sufficient space for an imaging run on our nodes, so it would be nice to have the option to change that in e.g. the input config file (unless that exists and I missed it) or if it follows the --tmpdir-prefix that is passed to cwltool/toil (not sure if that info is available in the workflow files).