Don't delete XComGame/Published by default in `clean` task

[robojumper]

As much as it is needed for a full clean, I think it's not something that should happen as a result of running clean for the Community Promotion Screen for example. I predict myself cursing quite a bit if this version of the clean task ends up in several other projects and removes Published regularly because copying TFCs actually takes ages.

[Xymanek]

Perhaps #18 is a better approach? It would support both full clean and a localized effect

[robojumper]

Right, I forgot about that. The only issue I see there is disk space: Most people have the CHL project on a different drive than the game, and maybe wouldn't expect their project directory to grow by 10GB.

[Xymanek]

Considering we are already planning to copy the TFCs over for asset cooking (#10), I think it's just an unfortunate price to pay. We just need to make sure to mention it somewhere prominent

[robojumper]

Perhaps a good way to mitigate that would be the addition of an option to override the BuildCache location.

[Xymanek]

Good idea, but it would need to be in some untracked file (i.e. not in build.ps1)