Cope with PRESSURE_VESSEL_SHARE_HOME=0 and unspecified home directory

@timo wants Help -> System Information to be able to succeed without Steam either resetting PRESSURE_VESSEL_SHARE_HOME or setting PRESSURE_VESSEL_HOME. This means we need to use something as the game-specific home directory, but we do not have a game ID.

We could use a tmpfs, but the big risk with a tmpfs is that if we accidentally get into that code path for a real game, then the user will not be aware that anything is wrong, but their configuration and saved games will be discarded.

Also, if we want to allow running non-Steam games in a container at some point, those don't have a game ID and so would be in the same situation as Help -> System Information. I don't think we want non-Steam games' configuration and saved games being silently lost.

I'd say maybe make it log a warning, but we don't want Help -> System Information to be triggering warnings.

Perhaps it's enough if we quietly allow it under PRESSURE_VESSEL_BATCH, but make it continue to be an error when not under PRESSURE_VESSEL_BATCH? That would mean Help -> System Information will succeed, and Proton games' setup commands will succeed even if we accidentally get onto that code path (for instance if Steam changes the way it communicates the game-ID to the game), but launching the actual game will fail.