fix(pkg): do not drop backtrace from user errors (#11457)

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

src/dune_pkg/opam_solver.ml

@@ -1760,7 +1760,7 @@ let solve_package_list packages ~context =
      | OpamPp.(Bad_format _ | Bad_format_list _ | Bad_version _) as bad_format ->
        (* CR-rgrinberg: needs to include locations *)
        User_error.raise [ Pp.text (OpamPp.string_of_bad_format bad_format) ]
-     | User_error.E _ -> reraise exn.exn
+     | User_error.E _ -> Exn_with_backtrace.reraise exn
      | _ ->
        Code_error.raise
          "Unexpected exception raised while solving dependencies"