How to skip a GitLabCI job from a before_script

yaml.yml

build-job:
  before_script:
    - |
      if [[ "${CI_COMMIT_BRANCH}" != "${CI_DEFAULT_BRANCH}" ]]; then
        git fetch origin $CI_DEFAULT_BRANCH
        SKIPPABLE_CHANGES=`git diff origin/$CI_DEFAULT_BRANCH --name-only $SKIP_SOURCE_STAGES_FOR_DIRS`
        ALL_CHANGES=`git diff origin/$CI_DEFAULT_BRANCH --name-only`
        if [[ $SKIPPABLE_CHANGES == "" ]]; then
          echo "There were no skippable changes, the script will continue"
        elif [[ $SKIPPABLE_CHANGES == $ALL_CHANGES ]]; then
          echo "All changes detected are inside a folder marked as skippable, the script will not continue"
          exit 0
        fi
      fi
  script:
    echo "Run some heavy script that doesnt depends on anything inside of SKIP_SOURCE_STAGES_FOR_DIRS"