Add ability to materialize WDL command environment to local machine

[unito-bot]

When a WDL job fails, I want to be able to inspect the environment it was working in.

I should be able to use toil debug-job to produce a directory on my machine with the WDL input files at their assigned paths, and also the command that the job is going to run.

┆Issue is synchronized with this Jira Story ┆Epic: Improve debugging experience ┆Issue Number: TOIL-1511