firefox: late-bind xdg-utils if broken

xdg-utils does not cross-compile.  Let's late-bind (using the
runtime $PATH) in this scenario so a native-built xdg-utils can be
used instead.

pkgs/applications/networking/browsers/firefox/wrapper.nix

@@ -303,7 +303,7 @@ let
           "''${executablePath}${nameSuffix}" \
             --prefix LD_LIBRARY_PATH ':' "$libs" \
             --suffix-each GTK_PATH ':' "$gtk_modules" \
-            --suffix PATH ':' "${xdg-utils}/bin" \
+            ${lib.optionalString (!xdg-utils.meta.broken) "--suffix PATH ':' \"${xdg-utils}/bin\""} \
             --suffix PATH ':' "$out/bin" \
             --set MOZ_APP_LAUNCHER "${launcherName}" \
             --set MOZ_SYSTEM_DIR "$out/lib/mozilla" \