Nixpkgs: Add a nixos-stable channel

[deliciouslytyped]

There are cases where its useful to be able to refer to the latest stable nixpkgs, and not need to manually update release numbers.

keywords: current stable

[prusnak]

What are these cases? Don't you want to update manually from old-stable to current-stable? Surely you are running stable for some reason, if you wanted rolling update you would be on unstable already.

[adisbladis]

This would be very valuable in CI for projects like poetry2nix & emacs-overlay where we support the latest stable release (whatever that happens to be at the time) & unstable.

[cole-h]

@prusnak I think the intention was moreso to allow scripting that wasn't so brittle as to only refer to a single release, but whatever the current release at the time is. Instead of referring to nixos-20.03 and then needing to update it ~6 months down the line, one could just "set-and-forget" nixos-stable.

[kisik21]

This might also simplify installation, as one might just add "nixos-stable" as a channel instead of figuring out which channel is actually stable. Setting system.stateVersion properly (the default generated configuration.nix does) should prevent most breakages that might be caused by automatically jumping to a next stable release.

This will also allow to use an older CD to always install a newer NixOS version automagically as long as the tooling stays similar and the user updates the channel with nix-channel --update (or was it --upgrade? I'm not using channels much, so I can't tell)

[veprbl]

https://discourse.nixos.org/t/why-is-there-no-rolling-stable-channel/3322/5

[FRidh]

This is a discussion for on discourse so closing.