nixos/openresty: fix build with nginx modules (#362348)

When adding modules to openresty, by either

- using `.override { modules = [ ..]; }` directly
- adding them to `services.nginx.additionalModules` or
- enabling e.g. something like `services.nginx.recommendedZstdSettings`

the build will then fail, as the actual `nginx` binary lands in
`nginx/bin/nginx` for openresty, and is only symlinked to `bin/nginx`
(and `bin/openresty`, for that matter).

This breaks the post-install script for nginx when removing references
to the aforementioned modules, since `remove-references-to` skips
symlinks. Thus, just read the symlink before in this case.

`readlink -fn` will read the symlink if it is one, otherwise just
returns the path itself.

The phase is also moved after the package-specific postInstall phase, at
that might move binaries around or create symlinks - as is the case for
openresty.

Can be easily reproduced using e.g.:

$ nix build --impure -E 'with import ./. {}; openresty.override { modules = [ nginxModules.zstd ]; }' -L

Signed-off-by: Christoph Heiss <christoph@c8h4.io>

pkgs/servers/http/nginx/generic.nix

@@ -206,8 +206,8 @@ stdenv.mkDerivation {
 
   postInstall =
     let
-      noSourceRefs = lib.concatMapStrings (m: "remove-references-to -t ${m.src} $out/bin/nginx\n") modules;
-    in noSourceRefs + postInstall;
+      noSourceRefs = lib.concatMapStrings (m: "remove-references-to -t ${m.src} $(readlink -fn $out/bin/nginx)\n") modules;
+    in postInstall + noSourceRefs;
 
   passthru = {
     inherit modules;