run: Mount original app on /run/parent/app when using --app-path=""

Before addressing CVE-2026-34078, we would always mount the original app
*somewhere*, either /app (in the normal case) or /run/parent/app (when
using a custom or empty /app for the subsandbox). The empty-app case
regressed during the fix for CVE-2026-34078; bring back previous behaviour.

Fixes: ac62ebe3 "run: Use O_PATH fds for the runtime and app deploy directories"
Resolves: https://github.com/flatpak/flatpak/issues/6568
Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit fde4716f67)

common/flatpak-run.c

@@ -3335,6 +3335,7 @@ flatpak_run_app (FlatpakDecomposed   *app_ref,
     }
   else if (custom_app_fd == FLATPAK_RUN_APP_DEPLOY_APP_EMPTY)
     {
+      original_app_target_path = "/run/parent/app";
       app_fd = -1;
       app_files = NULL;
     }