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...

Note that Ruby 2.6.8 fixes three severe vulnerabilities present in Ruby 2.6.7, while being backward compatible, so I don't see why you'd want that.