Add a flake.nix to all template to implement "inputOverrides" pattern

[jbgi]

Draft, depends on #18 and need pins update after #18 get merged.
Ready for review/merge.

[vic]

#18 merged now

[vic]

@jbgi one question: Is there any particular reason we are adding flake.nix to all templates ? This project is supposed for "setups without flake.nix", so maybe there's value of showing your override inputs pattern in the flake template only. Do you believe it is necessary for all templates to have a flake.nix ?

[jbgi]

@vic My main goal is to be able to override and/or follows inputs of any with-inputs-based project. That requires a convention for finding the ouputs function , thus using the flake one, as it is well-defined and I figured most project would expose a flake.nix anyway, for compatibility with flake.

But we could also support a non-flake way to inject inputsOverrides, by checking if a source has a default.nix exposing a function with all arguments being optional, including one being name inputsOverrides. It sounds a bit more fragile though... but I can open a PR to support this if you'd like?

[jbgi]

@vic actually found an issue with my input override via flake for some flake (eg. https://github.com/nix-community/nixpkgs.lib/blob/master/flake.nix). Opening a MR soon to switch to using default.nix as described above.