secondary users are not listed in polkit graphical prompt

[Didier Roche-Tolomelli]

Tested with 124-1ubuntu1 on noble

1. Create 3 users on the machine:
- sudo-user1, part of sudo group
- sudo-user2, part of sudo group
- normal-user, not part of the sudo group
2. Login with normal-user
3. Try to execute pkexec or any other command prompting for polkit
-> only sudo-user1 password auth is proposed, before, we had a listbox to select all eligible user
4. Remove sudo-user1
5. Try pkexec again:
-> now sudo-user2 is proposed, showing that the configuration for this user is correct.

It seems that only the first "admin" of the machine is taken into account. However, as normal-user, I may not have sudo-user1 around, but only sudo-user2.