Making nixos keep a ref to its stdenv

This will help a lot on ARM, where nix-collect-garbage erases gcc; then, any
change to a small system config file requires rebuilding gcc again.

I don't know why it does not happen on x86. Maybe it just pulls the gcc from
hydra, if garbage is collected.

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

@@ -64,6 +64,7 @@ let
       ln -s ${config.system.build.etc}/etc $out/etc
       ln -s ${config.system.path} $out/sw
       ln -s "$systemd" $out/systemd
+      ln -s ${pkgs.stdenv} $out/stdenv
 
       echo -n "$configurationName" > $out/configuration-name
       echo -n "systemd ${toString config.systemd.package.interfaceVersion}" > $out/init-interface-version