CI: default:artifacts:expire_in is ignored

Summary

When the expire_in value is defined in the default section, this is ignored by gitlab ci.

Steps to reproduce

Define a .gitlab-ci.yml like this and trigger the pipeline (see example project)

default:
  artifacts:
    expire_in: 1 day

stages:
  - expire

expire:default:
  image: galilee/php-tester:7.3
  stage: expire
  cache:
    paths:
      - .composer
  script:
    - composer install --prefer-dist --no-progress
  artifacts:
    paths:
      - vendor

expire:specific:
  image: galilee/node-build:12
  stage: expire
  cache:
    paths:
      - .npm
  script:
    - npm ci
  artifacts:
    paths:
      - node_modules
    expire_in: 12 hours