Default nix installer should select latest stable version, not latest unstable version

[Tristano8]

Description

The installer (in my case, for MacOs) selects the latest unstable version of nix, which leads to the following surprising behaviour

After installation:

$ nix --version
$ nix (Nix) 2.21.1
$ nix upgrade-nix --dry-run
$ warning: would upgrade to version 2.18.1

Expected behavior

The installed nix version should be the latest stable version as a default, maybe with the ability to specify the unstable version for the subset of users who want that

nix-env --version output nix-env (Nix) 2.21.1