Combiner TmpDir as a String, not Path

[MattWellie]

Purpose

• The temp-dir should be a String, so that we don't check for its existence
• It's intentionally deleted by the next Stage, so a re-run of later stages of the workflow would accidentally re-trigger running the combiner
• e.g. here

Proposed Changes

• Make the temp-directory Path a String, so its existence isn't checked
• Make the String into a Path object when it's used to generate temp output paths