[Feature]: Add environment variable check for disabling auto update on container start

### User Story
Add an additional check for an environment variable to the entrypoint-user.sh that allows to disable auto server updates on start.

### Game
Vintage Story

### Distro
Other

### Further Information
I am running a Vintage Story server with mods, some of them tend to break on (minor) updates of the game. As far as I can tell, there is no way to disable the auto update in the entrypoint-user.sh, except setting the VALIDATE_ON_START environment variable, which is kinda janky (it fails since validate only works for SteamCMD servers).