If you are on Linux and are the type to play a game arguably hours too long just to finish off the achievements list, I've found the perfect thing for you / your worst nightmare. Sentinel is a new tool you can download on GitHub that will add an achievements system to your games, even if you play them on a storefront other than Steam.
In fact, the creator "m0rpheus23" on Reddit has shared that its major use is to specifically work with games downloaded outside of Steam. In concept, it's quite a simple bit of software, really. Once downloaded, you simply choose to use the Steam Web API or an external data source (like SteamHunters), and the tool will grab achievement data there.
