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.
When upgrading bookstack, if something in the cache conflicts with the
new installation, the artisan commands might fail. To solve this, make
the cache lifetime bound to the setup service. This also removes the
`cacheDir` option, since the path is now handled automatically by
systemd.
