tests.devShellTools.nixos: Fix test

This increases the disk size, which I assume was consumed by the
various docker image operations.

nixos/tests/docker-tools-nix-shell.nix

@@ -17,7 +17,7 @@ in
       { ... }:
       {
         virtualisation = {
-          diskSize = 4000;
+          diskSize = 8000;
           docker.enable = true;
         };
       };