1. you need to add steam-run in your configuration.nix (see nixos wiki about steam)
2. change file: "..game/dontstarve64/bin/dontstarve.sh" and "..game/dontstarve32/bin/dontstarve.sh":
- export LD_LIBRARY_PATH=./lib64
+ export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:./lib64

(- means remove, + means add instead)
This solves problem: "./dontstarve: error while loading shared libraries: libGL.so.1: cannot open shared object file: No such file or directory"