Add option to ignore mods from auto-update

[chrisliebaer]

This pull request introduces support for skipping specific mods during the automatic update process by adding a new environment variable, UPDATE_IGNORE, allowing users to specify a comma-separated list of mod names that should not be updated. This helps in situations in which general utility mods can be updated without issues, but bigger mod-pack, especially from multiple developers require careful testing.

[chrisliebaer]

I would also like to take this opportunity to ask, whether you would add support for updating mods from external sources on a per-mod basis, since I always end up with a few patched mods where upstreaming is either not possible or not wanted by the developer. I'm fine with building this feature, but I would avoid doing so if you are against such additions.