utils.sh: When checking for LTS branch, use $TF_GERRIT_BRANCH

$GERRIT_BRANCH is explicitly unset by the build scripts, see
https://review.trustedfirmware.org/c/ci/tf-a-job-configs/+/3301 .

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I6f07faffe7318449df66a847b7eb95682a147939
diff --git a/utils.sh b/utils.sh
index fe52327..8627913 100644
--- a/utils.sh
+++ b/utils.sh
@@ -26,7 +26,7 @@
   if echo "$JENKINS_URL" | grep -q "oss.arm.com"; then
     source "$ci_root/arm-env.sh"
   elif echo "$JENKINS_URL" | grep -q "ci.trustedfirmware.org"; then
-    if echo "$GERRIT_BRANCH" | grep -q "lts-v2.8"; then
+    if echo "$TF_GERRIT_BRANCH" | grep -q "lts-v2.8"; then
       source "$ci_root/openci-lts-v2.8-env.sh"
     else
       source "$ci_root/openci-env.sh"