ExternalProject: Avoid unnecessary checkout on clone

The clone step checks out the cloned branch but is always followed by an
explicit checkout of the desired `GIT_TAG`.  Tell `git clone` not to
check out.
14 jobs for 2xCheckout_Attempt2
Status Name Job ID Coverage
  External
passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager