Provide some way to update non-nix-compliant snapshots

[madjar]

Some stuff (like the rustc or cargo snapshot) change everyday but stay at the same url, making it impossible to put them in nixpkgs.

I've written a dirty script that update the sha, but it could be possible to make it much more clean, using annotations like this:

11:  src = fetchurl {
12-    url = "some_url"; # nox-up:packagename
13-    sha256 = "some_sha"; # nox-up:packagename
14-  };

Making it possible for a nox-up script to find the relevant package, determine it's url, nix-prefetch it, update the sha, and build it.

The challenge would be with interpolated urls.

Some similar script could be used to update the ref of git packages.

[madjar]

Actually, the monitor (http://monitor.nixos.org/) kind of does that, so I should take a closer look.

[jagajaga]

Check this https://github.com/MarcWeber/nix-repository-manager

[madjar]

Looks great, thanks, I'll have to look into it.