Allow setting temp dir used by campaigns through flag/env var
Created by: mrnugget
This addresses a customer report in which src campaigns apply
couldn't
be run in CI because the docker-in-docker setup prohibits mounting
/tmp
into containers.
It also fixes parts of https://github.com/sourcegraph/src-cli/issues/316, since the temporary directory is also where the logs are stored, but we might want to split those locations up (in case a user wants to keep logs but not inside the temporary directory).