a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowBroken = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowBroken = true; }
to ~/.config/nixpkgs/config.nix.

cannot build derivation ‘/nix/store/6vi9vq0q7zibikzibr9nparh55s6gl00-nixos-vm.drv’: 1 dependencies couldn't be built
building path(s) ‘/nix/store/0fk02zgf3dxxfpxvqsha0llrfz1ahz77-nixos-system-webserver-18.09.git.527e97f’
building path(s) ‘/nix/store/39764nwhx9lrlswwi41f22aalxapwhaf-closure-info’
/nix/store/aj9rz4k3xvg8q9fzqfnb0q123707zp0n-builder: line 1: .attrs.sh: No such file or directory
builder for ‘/nix/store/h5h05qyfv4q18s2arkqmdbzgqgp7iqp0-closure-info.drv’ failed with exit code 1
cannot build derivation ‘/nix/store/fisdcq55lyprxz5zqjdy3bgs96in8nwa-run-nixos-vm.drv’: 1 dependencies couldn't be built
cannot build derivation ‘/nix/store/xwm6i10xlbafyq39xkxms1gn2xx1svxj-nixos-vm.drv’: 1 dependencies couldn't be built
cannot build derivation ‘/nix/store/wkv1v54j1vgdvpsmccj0a2fvhnrjj2x7-nixos-test-driver-acme.drv’: 3 dependencies couldn't be built
cannot build derivation ‘/nix/store/vsn70ll3bka8j8i24gv2f0hzcbf3m0mj-vm-test-run-acme.drv’: 1 dependencies couldn't be built
error: build of ‘/nix/store/vsn70ll3bka8j8i24gv2f0hzcbf3m0mj-vm-test-run-acme.drv’ failed