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

@@ -98,7 +98,7 @@ let
   # `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}";