f670e108

By: Greg Pfeil <greg@technomadic.org>

Replace the profile’s contents in one generation

`nix profile` requires one generation to clear the profile and another to add
the packages. `nix-env --set` can replace the contents in one generation, but
it’s a “legacy” command.

NixOS and Home Manager still use `nix-env` because of this
issue (NixOS/nix#6349).