The description for options.nixpkgs.system already hints at this:
Neither ${opt.system} nor any other option in nixpkgs.* is meant
to be read by modules and configurations.
Use pkgs.stdenv.hostPlatform instead.
We can support this goal by not elaborating the systems anymore, forcing
users to go via pkgs.stdenv.
This will prevent problems when making the top-level package sets
composable in the next commit. For this to work, you should pass a fully
elaborated system to nixpkgs' localSystem or crossSystem options.
Backport of Nixpkgs commit 0a19371146130c0e2a402fd0c35f8283b0e81910.
Co-authored-by: Wolfgang Walther <walther@technowledgy.de>
7c72c013b1