When we run a local tryjob, the code sync would be a LOT faster if we used git cache. This is a relatively minor change to "cros tryjob --local", just have it supply the --git-cache-dir option to cbuildbot_launch.

However, I'm not sure what directory should be used.

A few options:
  /tmp/git-cache
  $SANDBOX/.cache/git-cache (for the sandbox holding cros tryjob)
  $BUILDROOT/.cache/git-cache
  $BUILDROOT/.git-cache