tailslib: Fix spawn_tps_frontend

The three commits I mentioned in https://gitlab.tails.boum.org/segfault/tails-private/-/merge_requests/4#note_212663 didn't make to stable. I think I only pushed them to the branch in the tails repo and @boyska merged the branch from my tails-private repo.

This should fix the ASP scenarios "I set up Additional Software when installing a package without persistent partition and the package is installed next time I start Tails" and " Recovering in offline mode after Additional Software previously failed to upgrade and then succeed to upgrade when online" which currently fail on stable.