fix: hardcode png format

# You can access the resulting images by adding the

# following to a README.md:

# ![description](/../-/jobs/artifacts/master/raw/filename.png?job=docs%3Adrawio)

# Replace filename with the name of the .drawio file, and the extension

# if you changed the FORMAT variable.

# Replace filename with the name of the .drawio file.

# The ref (currently master) is hardcoded, so the README.md render will only

# show what is built on that ref.

  stage: build

  variables:

    EXTRA_ARGS: ""

    FORMAT: "png"

  script:

    - /opt/drawio-desktop/entrypoint.sh --remove-page-suffix --output . --format ${FORMAT} ${EXTRA_ARGS}

    - /opt/drawio-desktop/entrypoint.sh --remove-page-suffix --output . --format png ${EXTRA_ARGS}

  artifacts:

    paths:

      - "**/*.$FORMAT"

      - "**/*.png"