portal: Use --bind-fd, --app-fd and --usr-fd options to avoid races

Now that flatpak_run_app accepts fds for app and runtime deploy, as well
as bind and ro-bind fds, and flatpak-run exposes the functionality, we
can finally hook this all up to the flatpak portal!
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f