disko: get rid of impure imports

This is a backwards-incompatible change but it allows to pin nixpkgs,
which is desirable especially in professional environments where
reproduciblity is important.

example/default.nix

@@ -1,6 +1,10 @@
 # usage: nix-instantiate --eval --json --strict example | jq -r .
 
-with import ../lib;
+let
+  # TODO: get rid of NIX_PATH dependency here
+  pkgs = import <nixpkgs> {};
+in
+with import ../lib { inherit (pkgs) lib;};
 
 {
   config = config (import ./config.nix);