Enable s3 storage for runners

Now that minio s3 storage is setup it would probably be worth setting up the runner cache to use the s3 storage, which will make the cache available to all runners.

Its a fairly simple change to the runner's config.toml, something like the following works:

[[runners]]
  name = "xxx"
  output_limit = 10240
  url = "https://xxx"
  id = 0
  token = "xxx"
  token_obtained_at = 0001-01-01T00:00:00Z
  token_expires_at = 0001-01-01T00:00:00Z
  executor = "docker"
  pre_clone_script = "umask 022"
  pre_get_sources_script = "umask 022"
  [runners.cache]
    Type = "s3"
    Shared = true
    MaxUploadedArchiveSize = 0
    [runners.cache.s3]
      ServerAddress = "objects.torproject.org"
      AccessKey = "gitlab"
      SecretKey = "xxx"
      BucketName = "gitlab-runner"
      BucketLocation = "seattle"
  [runners.docker]
    tls_verify = false
    image = "docker:stable"
    privileged = true
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/certs/client", "/cache", "/run/docker.sock:/run/docker.sock"]
    shm_size = 0