scripts/clone.sh: fix bug

Previously, the refspec of tf-a-job-configs was fetched
but FETCH_HEAD was never checked out,
causing the master branch to always be used.

Change-Id: I1a53aa421de0c1df93c0128af095af0842fc0a89
Signed-off-by: Arthur She <arthur.she@linaro.org>
diff --git a/scripts/clone.sh b/scripts/clone.sh
index 3929317..688c75b 100755
--- a/scripts/clone.sh
+++ b/scripts/clone.sh
@@ -84,6 +84,7 @@
     git clone ${GIT_CLONE_PARAMS} ${GIT_REPO}/${JOBS_PROJECT} ${SHARE_FOLDER}/${JOBS_REPO_NAME}
     cd ${SHARE_FOLDER}/${JOBS_REPO_NAME}
     git fetch origin ${JOBS_REFSPEC}
+    git checkout FETCH_HEAD
 else
     cd ${SHARE_FOLDER}/${JOBS_REPO_NAME}
 fi