nixos/caddy: fix caddy fmt for caddy v2.6.3

The nixos module uses `caddy fmt` to *format* the generated Caddyfile.
v2.6.3 slightly altered `caddy fmt`'s behavior.

Excerpt from https://github.com/caddyserver/caddy/releases/tag/v2.6.3
> cmd: caddy fmt return code is 1 if not formatted
> cmd: fmt exit successfully after overwriting config file

Which is why the internal `Caddyfile-formatted` now uses `cp` and `caddy fmt --overwrite` instead

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

@@ -35,7 +35,8 @@ let
 
       Caddyfile-formatted = pkgs.runCommand "Caddyfile-formatted" { nativeBuildInputs = [ cfg.package ]; } ''
         mkdir -p $out
-        ${cfg.package}/bin/caddy fmt ${Caddyfile}/Caddyfile > $out/Caddyfile
+        cp --no-preserve=mode ${Caddyfile}/Caddyfile $out/Caddyfile
+        caddy fmt --overwrite $out/Caddyfile
       '';
     in
       "${if pkgs.stdenv.buildPlatform == pkgs.stdenv.hostPlatform then Caddyfile-formatted else Caddyfile}/Caddyfile";