allow use of flake git ref?

[Rafmaninoff]

Hello, I often forget to git pull before building, so, I was trying to make nh only build from github, like you can with nixos-rebuild switch --flake github:user/repo, however, programs.nh.flake must be of type path.

[viperML]

You can manually set the FLAKE environment variable in different ways, such as your shell config or environment.sessionVariables