nginx: expose list of additional modules (#53897)

Currently, it seems there is no easy way to override package to add
modules. For example, if we want to add the `ipscrub` module, we can
do:

    pkgs.nginxStable.override {
      modules = [ pkgs.nginxModules.ipscrub ];
    };

But, then, we loose `rtmp`, `dav` and `moreheaders` which are defined
in `all-packages.nix`. With this modification, we can now do:

    pkgs.nginxStable.override {
      modules = pkg.nginxStable.passthru.modules ++ [ pkgs.nginxModules.ipscrub ];
    };

pkgs/servers/http/nginx/generic.nix

@@ -87,6 +87,8 @@ stdenv.mkDerivation {
     mv $out/sbin $out/bin
   '';
 
+  passthru.modules = modules;
+
   meta = {
     description = "A reverse proxy and lightweight webserver";
     homepage    = http://nginx.org;