Avoid changing the source directory during busytown builds

During a busytown build all overlays are now applied
in a temp directory.

Test: python build_busytown_test.py \
    && python container_test.py && python nsjail_test.py && \
    python overlay_test.py
Change-Id: Ib1db0f7294c635c8e925875b38bede8318041618