Campaign mission briefing runs every time a mission is loaded from savegame

[side2k]

Describe the bug

When savegame from campaign mission is loaded, mission briefing is show, even if the save game was created after victory.

To Reproduce

1. Start any campaign mission
2. Play a bit
3. Save the game
4. Quit the game
5. Load the previously saved game
6. See a mission briefing

Expected behavior

No briefing should be shown after savegame is loaded.

Screenshots and Logs

Video from #392 shows this perfectly.

Desktop (please complete the following information):

• OS: Kubuntu
• Version 21.04