nix: introduce fake nixpkgs (#9)

Since <nixpkgs> wasn't working, might as well make the error more
explicit. That way, users are quickly aware if they forgot to pin their
nixpkgs.

images/nix/fake_nixpkgs/default.nix

@@ -0,0 +1,7 @@
+_:
+throw ''
+This container doesn't include nixpkgs.
+
+Pin your dependencies. Or if you must, override the NIX_PATH environment
+variable with eg: "NIX_PATH=nixpkgs=channel:nixos-unstable"
+''