Fix Instant Action mode not always applying the correct (random) number of bots if...

Fix Instant Action mode not always applying the correct (random) number of bots if started while playing the campaign:

* example where cvar_set doesn't work: start Xonotic, set bot_number to 0, start a campaign level, start an Instant Action game: bot_number is set to 0
* example where cvar_set works: start an Instant Action game twice, start a campaign level, start an Instant Action game: bot_number is correctly set