This speeds up the `niv init` command by getting rid of
`nix-prefetch-url` calls (by default). In particular:
* `niv` itself is not added anymore
* `nixpkgs` is fetched from a "known" version
Here we make sure that some environment variables are prepended
(GITHUB_PATH, GITHUB_INSECURE). For consistency's sake, other
environment variables (GITHUB_TOKEN, etc) _can_ also be prepended with
NIV_. This also issues a warning if `GITHUB_PATH` or `GITHUB_INSECURE`
is set.
This adds a few configuration options to `niv init`:
* `--no-nixpkgs`: skips the import of nixpkgs
* `--nixpkgs-branch`: specifies the branch to use for nixpkgs
* `--nixpkgs`: specifies the repo to use for nixpkgs
NOTE: this changes the default nixpkgs branch from nixos-19.09 to
release-19.09.
