Allow selecting nixpkgs source on devenv init

[pan93412]

Description

As cachix/devenv-nixpkgs has been lagging for 3 months (due to cachix/devenv-nixpkgs#2), I find myself needing to update the URL of the nixpkgs source to github:nixos/nixpkgs/nixpkgs-unstable whenever I initiate devenv.

It would be great to have the option to edit this field with an argument for devenv init.

Alternative Solution

I have tried override-inputs but it does not work.

devenv init -o nixpkgs github:nixos/nixpkgs/nixpkgs-unstable

[pan93412]

https://github.com/cachix/devenv/blob/main/devenv/init/devenv.yaml

Seems like it is hard-coded. override-inputs is not for devenv init. Just close it now since I can also write a simple script to sed it after devenv init, and it is not really important at all.