Use docbook-xsl-ns with man opt dedup patch

Should fix duplicate option names into the manpages generated with nmd.
See:
- NixOS/nixpkgs#166304
- NixOS/nixpkgs#166509
- nix-community/home-manager#2820

lib/manual-docbook.nix

@@ -31,7 +31,9 @@ with lib;
 
 let
 
-  inherit (pkgs) docbook5 docbook-xsl-ns;
+  inherit (pkgs) docbook5;
+  # See https://github.com/NixOS/nixpkgs/pull/166509
+  docbook-xsl-ns = pkgs.docbook-xsl-ns.override { withManOptDedupPatch = true; };
 
   docBookFromAsciiDocDirectory = pkgs.runCommand "converted-asciidoc" {
     nativeBuildInputs = [ (getBin pkgs.asciidoc) (getBin pkgs.libxslt) ];