Fix existing game install not updating

[Diyagi]

After seeing a few reports about the game not updating when the container restarts, I looked into it and found it was caused by an oversight in the script that sets up the game files. This PR fixes that.

The issue was that running validation doesn't actually update the game files. The fix is to check if the game is already installed: if it is, we run SteamCMD/DepotDownloader without validation so updates can be applied; if not, we run it with validation for the initial install.

[Diyagi]

Turned this into a draft, figured out why updates are failing, and it has to do with https://steamdb.info/sub/17906/apps/
The anonymous user only has full access to the apps in that package, anything not in it SteamCMD only access the latest manifest, even if its free.
Since the container does not make the SteamCMD manifest cache persistent, when an update releases, it loses access to the current install manifest and cannot fetch it for the update.
I will look into a better solution for this later.