Really fix SetInfectedAsync's packet structure

[libhalt]

Description

New Impostor list is prefixed with the list's length not the host's netid

Use Case & Test
https://gist.github.com/libhalt/e3f98c717a29514dcc259c6ca9c7f708

[js6pak]

Even if the packet is now correct, are you sure your use case is even working? (without any side effects)

[libhalt]

The above test plugin has been tested locally on itch version of the game with 5 locally running clients.
(the game end screen showed the correct impostor, only the impostor had red name tag , etc)

[Elsensee]

He meant, it's usually not possible to set the impostors without client-side modifications.
Either the host would be de-synced or the clients would ignore the change (depending on when you send the SetInfectedRPC)

[libhalt]

I agree with your/his statement that this only works at a very specific situation but it is usefull at that very spefic situation. Initially, i thought about limiting the call to during IGameStartingEvent but that is a "breaking change" that would require edit of the doc (although i cant imagine any plugin using this method outside the limit that will be imposed as it would be non functional) and outside the scope of this PR. This PR is focused on fixing the packet structure and I thought that by focusing on fixing the packe structure this change would be merged easier as their were previous attempts at fixing the packet structure despite the known limitation (2ea270d) . However, if the maintainers wish this method to be limited I can add that in