cannot define runner builds_dir to be "/"

im trying to configure the runner to always clone the repo to the "/" dir, and not to the default "/builds".

the config.toml:

[[runners]]
    name = "***"
    url = "***"
    token = "***"
    tls-ca-file = "***"
    executor = "kubernetes"
    environment = ["DOCKER_TLS_CERTDIR="]
    output_limit = "***"
    builds_dir = "/"
    [runners.custom_build_dir]
      enabled = true

but it does not work, the pod created by the runner shows this error:

Error: Error response from daemon: invalid volume specification: '/var/lib/kubelet/pods/<some-id>/volumes/kubernetes.io~empty-dir/repo:/': invalid mount config for type "bind": invalid specification: destination can't be '/'

I also tried builds_dir = "/builds/.." (which gave the same kind of error) and builds_dir = "" (which was totally ignored, the clone path remained "/builds")

I also tried override the "CI_BUILDS_DIR" and "GIT_CLONE_PATH" vars with "/" and "/", and it did not help...

how can I configure the builds_dir to be "/" ?