Add stdenvNoCC

This is a standard environment that doesn't contain a C/C++
compiler. This is mostly to prevent trivial builders like runCommand
and substituteAll from pulling in gcc for simple configuration changes
on NixOS.

nixos/modules/system/activation/top-level.nix

···
   # `switch-to-configuration' that activates the configuration and
   # makes it bootable.
   baseSystem = showWarnings (
-    if [] == failed then pkgs.stdenv.mkDerivation {
+    if [] == failed then pkgs.stdenvNoCC.mkDerivation {
       name = let hn = config.networking.hostName;
                  nn = if (hn != "") then hn else "unnamed";
           in "nixos-system-${nn}-${config.system.nixosLabel}";

nixos/modules/system/etc/etc.nix

···
 
   etc' = filter (f: f.enable) (attrValues config.environment.etc);
 
-  etc = pkgs.stdenv.mkDerivation {
+  etc = pkgs.stdenvNoCC.mkDerivation {
     name = "etc";
 
     builder = ./make-etc.sh;

pkgs/build-support/substitute/substitute-all.nix

···
-{ stdenv }:
+{ stdenvNoCC }:
 
 args:
 
 # see the substituteAll in the nixpkgs documentation for usage and constaints
-stdenv.mkDerivation ({
+stdenvNoCC.mkDerivation ({
   name = if args ? name then args.name else baseNameOf (toString args.src);
   builder = ./substitute-all.sh;
   inherit (args) src;

pkgs/top-level/all-packages.nix

···
     else
       stdenv);
 
+  stdenvNoCC = stdenv.override { cc = null; };
+
   # For convenience, allow callers to get the path to Nixpkgs.
   path = ../..;