Restore warning about 'nix' being experimental
Restore warning about 'nix' being experimental

Fixes #4552.

rycee/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix