Update .gitlab-ci.yml file

.build-reference-and-dut-binaries: &build-reference-and-dut-binaries

  - current_commit_sha=$(git rev-parse HEAD)

  ### build reference binaries

  - echo "building reference branch: $REFERENCE_BRANCH"

  - echo "building reference branch $REFERENCE_BRANCH"

  - git checkout $REFERENCE_BRANCH

  - git pull

  - make clean