fixup! backend: Dedup some code for temporary auth subjects

See https://bugs.freedesktop.org/show_bug.cgi?id=23867
which turned up from `git annotate` here.

I was reading this code and noticed it was duplicated.
1 job for walters-polkit-dedup-subj-conversion in 1 minute and 56 seconds (queued for 2 seconds)