It's cause you're not actually supposed to use nix-env: https://stop-using-nix-env.privatevoid.net/
You're actually supposed to be using nix search nixpkgs#packagename
to search and nix profile install nixpkgs#packagename
to install.
However, to use both of those, you need to have the "experimental" (not really though, most of the community uses them) features of nix-command and nix flakes enabled, which they aren't by default.
And of course, nowhere on the main documentation did I find any if that, I only found it via the pain of using it wrong, and forum posts.
Nix's documentation is horrific. I've had situations where I only got help via discord...
Did some searching, also can't find the original forum post lol.
https://github.com/nix-community/nixGL/issues/114#issuecomment-1585323281
and this: https://pmiddend.github.io/posts/nixgl-on-ubuntu/
The latter looks like what I originally used, but what I originally used broke the generated application menu entries.