doc: don't give nix users bad advice

See [here][0]. Plus `nix profile` and imperative package management in
general is usually the wrong thing to do when using Nix.

Nix users should be able to reference existing documentation for how to
install things; this repository should simply say where it's packaged
and leave it up to the user to decide how they want to install it.

[0]: https://stop-using-nix-env.privatevoid.net/

README.md

@@ -121,19 +121,8 @@ sudo apt install -y eza
 
 [![nixpkgs unstable package](https://repology.org/badge/version-for-repo/nix_unstable/eza.svg)](https://repology.org/project/eza/versions)
 
-Eza is available from [Nixpkgs](https://github.com/NixOS/nixpkgs).
-
-For `nix profile` users:
-
-```shell
-nix profile install nixpkgs#eza
-```
-
-For `nix-env` users:
-
-```shell
-nix-env -i eza
-```
+Eza is available from [Nixpkgs](https://github.com/NixOS/nixpkgs) and from the
+flake in this repository.
 
 ### Gentoo