nixos/tests/installer: Add stdenvNoCC to extraDependencies

The installer tests are failing after 505e94256e
due to `nixos-rebuild switch` in the installed system trying to build
stdenvNoCC.

Seems that previously, stdenvNoCC wasn't in the installed
system either, but all the direct dependencies for the build were
(I don't really understand why, for that matter), so the building
actually went fine and everything worked.

But now gcc is also a direct build dependency due to allowedRequisites
containing gcc (even though it doesn't become a runtime dependency)
which doesn't get to the installed system.

All in all, let's ensure stdenvNoCC actually gets to the installed
system. It's after all necessary in almost any NixOS config build.

nixos/tests/installer.nix

@@ -18,6 +18,9 @@ let
             <nixpkgs/nixos/modules/testing/test-instrumentation.nix>
           ];
 
+        # To ensure that we can rebuild the grub configuration on the nixos-rebuild
+        system.extraDependencies = with pkgs; [ stdenvNoCC ];
+
         ${optionalString (bootLoader == "grub") ''
           boot.loader.grub.version = ${toString grubVersion};
           ${optionalString (grubVersion == 1) ''