The `extraConfig` parameter only handles text - it doesn't support
arbitrary secrets and, with the way it's processed in the setup
script, it's very easy to accidentally unescape the echoed string and
run shell commands / feed garbage to bash.
To fix this, implement a new option, `config`, which instead takes a
typed attribute set, generates the `.env` file in nix and does
arbitrary secret replacement. This option is then used to provide the
configuration for all other options which change the `.env` file.