Yeah this is very likely OP’s issue. You can test with:
nix run -f channel:nixos-20.03 glxinfo -c glxgears
nix run -f channel:nixos-unstable glxinfo -c glxgears
If you’re running NixOS 20.03 the latter should give the same error as in OP. So I guess we have two choices: roll back the user channel, or upgrade the system. I opted for the former by
nix-channel --add https://nixos.org/channels/nixos-20.03 nixpkgs
nix-channel --add https://nixos.org/channels/nixos-unstable unstable
nix-channel --update
I’ve also pinned my home-manager Nixpkgs as well so it’s more obvious when a channel upgrade breaks something.