nixos/installation-device: don’t include stdenv (#418267)

nixos/modules/profiles/installation-device.nix

···
     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

···
               system.extraDependencies =
                 with pkgs;
                 [
+                  # TODO: Remove this when we can install systems
+                  # without `stdenv`.
+                  stdenv
+
                   bintools
                   brotli
                   brotli.dev