xapian-omega: provide xapian template directory

These templates are kinda important for the default `omega` CGI server
configuration to work.

pkgs/development/libraries/xapian/tools/omega/default.nix

@@ -12,6 +12,11 @@ stdenv.mkDerivation rec {
   buildInputs = [ xapian perl pcre2 zlib libmagic ];
   nativeBuildInputs = [ pkg-config ];
 
+  postInstall = ''
+    mkdir -p $out/share/omega
+    cp -r templates $out/share/omega
+  '';
+
   meta = with lib; {
     description = "Indexer and CGI search front-end built on Xapian library";
     homepage = "https://xapian.org/";