flake: export lib with references to nixpkgs input

This previously pointed at `<nixpkgs/...>`, which might be anything. It
caused this error for me, when I tried to use `lib.makeDiskoTest`
downstream:

  error: cannot look up '<nixpkgs/nixos/tests/make-test-python.nix>' in
  pure evaluation mode (use '--impure' to override)

flake.nix

@@ -24,6 +24,9 @@
 
       diskoLib = import ./lib {
         inherit (nixpkgs) lib;
+        makeTest = import (nixpkgs + "/nixos/tests/make-test-python.nix");
+        eval-config = import (nixpkgs + "/nixos/lib/eval-config.nix");
+        qemu-common = import (nixpkgs + "/nixos/lib/qemu-common.nix");
       };
     in
     {