In order to upgrade Haskell stack tool on NixOS without enabling nixos-unstable
for all packages it is possible to install just one single package from unstable channel like following:
nix-env -f https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz -iA stack
If it happens that it is still older version, try unstable-small channel instead which gets updated more often:
nix-env -f https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable-small.tar.gz -iA stack