Automatically allow workspace mounts in `valid_volumes` for the mounts done via `bind_workdir`, this obsoletes the need for `/workspace/**` or other insecure configurations which would expose workspaces between tasks.
Reviewed-on: https://gitea.com/gitea/runner/pulls/1203
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>