Modify extended job variable on the fly through script

I have the following jobs:

.build-image:
  extends: .build-docker-image
  stage: build
  variables:
    COMPOSER_HOME_DIR: /kaniko/.composer
    BUILD_ARGS: >
      COMPOSER_HOME=${COMPOSER_HOME_DIR}
      COMPOSER_CACHE_READ_ONLY=1
    ADD_KANIKO_ARGS: >
      --reproducible=${REPRODUCIBLE},
      --skip-unused-stages=true,
      --target=${BUILD_TARGET}
    BUILD_ARGS: >
      BUILD_VERSION=${BUILD_VERSION},
    REPRODUCIBLE: "false"
  rules:
    - *push_to_main_branch
    - *merge_request
  id_tokens: !reference [ .gcp_jwt_token, id_tokens ]
  before_script:
    - mkdir -p $COMPOSER_HOME_DIR
    - echo "${COMPOSER_AUTH}" > $COMPOSER_HOME_DIR/auth.json

build-app-image:
  stage: release
  extends: .build-image
  rules:
    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
      when: on_success
    - if: '$CI_MERGE_REQUEST_ID'
      changes:
        - src/**/*
      when: on_success
  variables:
    BUILD_TARGET: production

  before_script:
    # What I would like to happen
    - export COMPOSER_LOCK_HASH=$(sha1sum composer.lock | cut -d' ' -f1)
    - export YARN_LOCK_HASH=$(sha1sum yarn.lock | cut -d' ' -f1)
    - export PREBUILT_APP_IMAGE="europe-docker.pkg.dev/${ORG_PROJECT}/blabla/blabla-deps:sha-${COMPOSER_LOCK_HASH}"
    - export PREBUILT_ASSETS_IMAGE="europe-docker.pkg.dev/${ORG_PROJECT}/mcp/blabla-other-deps:sha-${YARN_LOCK_HASH}"
    - export BUILD_ARGS="COMPOSER_HOME=${COMPOSER_HOME_DIR}, APP_IMAGE_REF=${PREBUILT_APP_IMAGE},ASSETS_IMAGE_REF=${PREBUILT_ASSETS_IMAGE}"
    - !reference [ ".build-image", "before_script" ]

See the comment in the 2nd job, in its before_script section. In practice I have a build-app-image that is extending a .build-image which on its own is extending other jobs defined company-wide. These are defined company-wide in order to have them reusable, and they use Kaniko to run the Dockerfile instructions. They accept a BUILD_ARGS variable that they use for the Dockerfile. My build-app-imageis a job in my repo and the logic I am trying to make is that I calculate on the fly some image tags and I would like these tags to be injected in theBUILD_ARGS, since BUILD_ARGSis the one that gets passed in the Dockerfile. However, if I definedBUILD_ARGSas a variable in mybuild-app-imageI cannot override its value on-the-fly through the logic in mybefore-script. Is there a way therefore to modify a variable on the fly dynamically? Export` won't work either because it declares the variable shell-wise and not Gitlab CI/CD wise.