Add option to specify nixpkgs path or channel

[davidak]

I need nixpkgs with a patch, so this feature whould be useful!

Or can i already use an env var? Then, please document it.

Currently, i use nix-build like this:
[davidak@ethmoid:~/code/nixpkgs/nixos]$ time nix-build /home/davidak/code/nixpkgs/nixos -A config.system.build.isoImage -I nixos-config=/home/davidak/code/nixos-config-greenbone/machines/targets-image/configuration.nix default.nix

[zimbatm]

Fixed in e657889. You can now pass -I nixpkgs=/path/to/nixpkgs like on a nix-build command.

[zimbatm]

before that you could already do that with NIX_PATH=nixpkgs=/path/to/nixpkgs nixos-generate ...

[davidak]

Perfect. Thanks for the fast fix!

Now i can just specify my unmerged PR branch to build the image:

nixos-generate -I nixpkgs=https://api.github.com/repos/davidak/nixpkgs/tarball/backport_nixos-containers-TimeoutStartSec -c /home/davidak/code/nixos-config-greenbone/machines/targets-image/configuration.nix -f iso