This update makes using flake (self or external) modules and overlays
more explicit, by setting nixpkgs options directly in the config,
instead of the flake. To help import your own overlays and modules, the
`outputs` attribute is now available on your configs.
This changes how the templates use nixpkgs. Before, we instantiated
nixpkgs in the flake and passd it directly to the configs; this was good
for consistency, but silently ignored `nixpkgs.*` options, generating
confusion among users. We now use modules to configure nixpkgs.
This should help avoid cases such as:
https://discourse.nixos.org/t/allowunfree-doesnt-work-with-flake-managed-system/21798/17
The drawback is that we can no longer test overlayed packages like so:
`nix build .#foo`, you can do it like so instead: `nix build
.#nixosConfigurations.your-system.pkgs.foo`.
Personally, I've been avoiding override packages globally in my own
config.
