floss: Update symlinks when re-running --run-bootstrap

If re-running --run-bootstrap multiple times, do everything except git
clone on re-runs. This makes it possible to switch between multiple
repos when developing while sharing the staging directory at ~/.floss.

Bug: 207009162
Tag: #floss
Test: ./build.py --run-bootstrap
Change-Id: Iba7324a3ab88ae47cf275e1492395508135ff76d