An important idea around the rust stuff in lib.systems is that it's
elaborated — this means that it should idempotently add to the values
passed in, if any.  But we missed that the names used for the
parameter and the elaborated value for "rustcTarget"/"config" didn't
line up.  The intention was to use "rustcTarget" everywhere in the new
interface, as a more descriptive name than "config".

This fixes setting the system in NixOS configuration, which results in
an already elaborated system being elaborated again.  Before, this
wouldn't produce the correct result:

% nix-instantiate --eval -A stdenv.hostPlatform.rust.rustcTarget --system armv7l-linux
"armv7-unknown-linux-gnueabihf"
% NIX_PATH= nix-instantiate --eval -E '(import nixos/lib/eval-config.nix { system = "armv7l-linux"; modules = []; }).pkgs.stdenv.hostPlatform.rust.rustcTarget'
"arm-unknown-linux-gnueabihf"

Fixes: e3e57b8 ("lib.systems: elaborate Rust metadata")
Fixes: #271000
(cherry picked from commit 4d23987)