campaigns: use the temp dir when mounting scripts in volume mode
Created by: LawnGnome
@eseliger noticed this one: volume workspaces were mounting their run scripts from the OS temporary directory, which could cause problems on macOS 11 + Docker 3.1. (And probably other configurations, but it worked for me on macOS 10.15.)
It should be using the same temporary directory as everything else, so let's do that.