nextcloud: Fails to install in 24.21

From https://discuss.freedombox.org/t/failed-to-install-nextcloud-after-upgrade-to-fb-24-21/3305

During installation, all commands run with 'podman exec --user=www-data' fail with the error:

Error: unable to find user www-data: no matching entries in passwd file

This seems to be a regression from the PrivateTmp=yes change we did in 21f6c912.