The default config for automatic ACME no longer works in Caddy <2.8.0.
This is due to changes with ZeroSSL's auth. Update to unstable Caddy
which is new enough to renew certs again.
Context: https://github.com/caddyserver/caddy/releases/tag/v2.8.0
Add `pkgs.unstable` as an overlay as recommended on the NixOS wiki. This
is needed here as Caddy must be runnable on all architectures.
