run-task: save destination checkout in an env

[ahal]

Imported from: https://bugzilla.mozilla.org/show_bug.cgi?id=1375496

When using the --vcs-checkout flag in run-task, we should save the checkout dir in an environment variable so consumers don't need to hardcode it somewhere.

[gshiva53]

Hi, I have created a PR #564 to try and fix the issue. I would love to know if I have successfully fixed the issue.