nixos/caddy: ensure vhosts come after user-specified cfg.extraConfig

This solves an issue, where loading the nixos-unstable module in
nixos-stable using `disabledModules` and `imports` resulted in the
following Caddyfile:

```
<globalConfig>

<vhosts>

<extraConfig>
```

instead of

```
<globalConfig>

<extraConfig>

<vhosts>
```

This is important in cases where `cfg.extraConfig` contains so called
Caddyfile snippets.

See https://caddyserver.com/docs/caddyfile/concepts#structure

Co-authored-by: Lin Jian <me@linj.tech>

nixos/modules/services/web-servers/caddy/default.nix

@@ -36,6 +36,7 @@ let
             ${cfg.globalConfig}
           }
           ${cfg.extraConfig}
+          ${concatMapStringsSep "\n" mkVHostConf virtualHosts}
         '';
 
         Caddyfile-formatted = pkgs.runCommand "Caddyfile-formatted" { nativeBuildInputs = [ cfg.package ]; } ''
@@ -340,7 +341,6 @@ in
       groups = config.users.groups;
     }) acmeHosts;
 
-    services.caddy.extraConfig = concatMapStringsSep "\n" mkVHostConf virtualHosts;
     services.caddy.globalConfig = ''
       ${optionalString (cfg.email != null) "email ${cfg.email}"}
       ${optionalString (cfg.acmeCA != null) "acme_ca ${cfg.acmeCA}"}