monado: use absolute runtime path in manifest

It used a weird mangled relative path that didn't work before and we have the
luxury that absolute paths always work exactly as we want them with Nix.

pkgs/applications/graphics/monado/default.nix

@@ -67,6 +67,7 @@ stdenv.mkDerivation rec {
 
   cmakeFlags = [
     "-DXRT_FEATURE_SERVICE=${if serviceSupport then "ON" else "OFF"}"
+    "-DXRT_OPENXR_INSTALL_ABSOLUTE_RUNTIME_PATH=ON"
   ];
 
   buildInputs = [