Add WL_TARGET_BRANCH for justifier.py

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Current Behavior

justifier.py in the csv-output stage assumes the whitelist repo branch to pull is master.

Purpose

Non-configurable branch for the dccscr-whitelist repo means our test projects will fail (since we don't want to merge the test projects into master)

Plan

Update justifier.py to be passed the WL_TARGET_BRANCH env var we've been using with this issue has popped up elsewhere

Acceptance Criteria

• justifier.py uses WL_TARGET_BRANCH to determine what branch of dccscr-whitelists to clone.