Some minor fixes.

Motivation for this change

Minor fixes:

• nix-env: Fix regression where completing some option arguments offers up garbage
• nix-env: Only complete main operators until one is supplied on the command line
• nix: Don't run completion if nix isn't found in path

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.