Here is an example if you absolutely want to pin Ruby to 2.6.7 while pulling all other packages from a more recent set of packages https://gist.github.com/Arkham/31297c1d75fc59f723a50063bc0f5...
I found the nixpkgs version using this tool: https://lazamar.co.uk/nix-versions/?channel=nixpkgs-unstable...