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
1 file changed