title: GitLab CI : merge or replace cache?
body: <p>I'm using GitLab CI.</p>
  
  <p>I have 2 jobs in the build stage that build differently my app. The 2 jobs upload a cache for the branch. I use the compiled sources to launch some tests in the test stage.</p>
  
  <pre><code>build:
    stage: build
    script:
      - ./gradlew build --build-cache --quiet
    cache:
      key: ${CI_COMMIT_REF_SLUG}
      paths:
       - "*/build"
  
  build_with_different_conf:
    stage: build
    script:
      - ./gradlew buildDiff --build-cache --quiet
    cache:
      key: ${CI_COMMIT_REF_SLUG}
      paths:
       - "*/build"
  
  Test:
    stage: test
    script:
      - ./gradlew test --build-cache
  </code></pre>
  
  <p>In my example, the job build_with_different_conf take more time to finish.</p>
  
  <p>My question is :  Is the last finishing build job upload the cache and replace the cache from the first build job or is it merging files whith the precedent job ?</p>
  
  <p>Thanks.</p>