Motivation for this change

It's well documented, and evident to anyone who spends any amount of time
helping people on IRC, that nix-env is a huge foot-gun. In an effort to limit
beginners exposure to this, I think we should not let command-not-found
suggest it, and instead tell people to use nix-shell.

This should help avoid a lot of frustration for beginners and save those who
provide support on IRC from diagnosing nix-env misuse constantly.

cc. @grahamc

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.