nix-env should respect <nixpkgs>

[domenkozar]

Extracted from #968:

If one has no channels, NIX_PATH=nixpkgs=http://nixos.org/channels/nixos-16.03/nixexprs.tar.xz should still provide packages to nix-env.

[danbst]

linking #886 and NixOS/nixpkgs#352

[danbst]

Also, linking part of NixUI proposal

nix-channel and ~/.nix-defexpr are gone. We'll use $NIX_PATH (or user environment specific
overrides configured via nix set-path) to look up packages. Since $NIX_PATH supports
URLs nowadays, this removes the need for channels: you can just set $NIX_PATH to e.g.
https://nixos.org/channels/nixos-15.09/nixexprs.tar.xz and stay up to date automatically.

[edolstra]

Indeed this is now implemented (d74236d). We probably shouldn't mess with legacy nix-env semantics though.

[CMCDragonkai]

@edolstra Was this only implemented for the new nix tool (and only for nix-build?) and not the current nix-env?

Oh it looks like it's not even part of current nix-env :-(.

[edolstra]

@CMCDragonkai Yeah, only the new interface.

[uri-canva]

I just tested export NIX_PATH=nixpkgs=$HOME/github/NixOS/nixpkgs with nix-env (Nix) 2.0 and it worked, it picked up my local changes to the packages.

Is this intentional or is it a bug? #993 (comment) makes it sound like the behaviour of nix-env with regards to NIX_PATH was not going to change.

[edolstra]

As far as I know the behaviour hasn't changed.

[CMCDragonkai]

@edolstra I'm wondering why the nix 2.0 doesn't have a nix-env replacement such that it respects the NIX_PATH?

[edolstra]

@CMCDragonkai Just that I haven't implemented it yet (mainly because it's TBD what to do wrt declarative vs. imperative package management and channels). There are some notes at https://gist.github.com/edolstra/efcadfd240c2d8906348.