nixos/installation-device: don’t include stdenv

A basic NixOS system shouldn’t require `stdenv` to install, and
it adds something like 45 MiB of build tools to the installer
closure. This doesn’t really help much at present since
WebKitGTK(?!) has a dodgy reference to GCC, but it’s a step in the
right direction.

nixos/modules/profiles/installation-device.nix

@@ -107,11 +107,10 @@ with lib;
     boot.kernel.sysctl."vm.overcommit_memory" = "1";
 
     # To speed up installation a little bit, include the complete
-    # stdenv in the Nix store on the CD.
+    # stdenvNoCC in the Nix store on the CD.
     system.extraDependencies =
       with pkgs;
       [
-        stdenv
         stdenvNoCC # for runCommand
         busybox
         # For boot.initrd.systemd

nixos/tests/installer.nix

@@ -709,6 +709,10 @@ let
               system.extraDependencies =
                 with pkgs;
                 [
+                  # TODO: Remove this when we can install systems
+                  # without `stdenv`.
+                  stdenv
+
                   bintools
                   brotli
                   brotli.dev