default nixpkgs branch is 20.05

[Solène Rapenne]

hello, using niv 0.2.19 from NixOS on unstable channel, when I use niv init, I get the following code in nix/sources.json

{
    "niv": {
        "branch": "master",
        "description": "Easy dependency management for Nix projects",
        "homepage": "https://github.com/nmattia/niv",
        "owner": "nmattia",
        "repo": "niv",
        "rev": "5830a4dd348d77e39a0f3c4c762ff2663b602d4c",
        "sha256": "1d3lsrqvci4qz2hwjrcnd8h5vfkg8aypq3sjd4g3izbc8frwz5sm",
        "type": "tarball",
        "url": "https://github.com/nmattia/niv/archive/5830a4dd348d77e39a0f3c4c762ff2663b602d4c.tar.gz",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
    },
    "nixpkgs": {
        "branch": "release-20.03",
        "description": "Nix Packages collection",
        "homepage": "",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "eb73405ecceb1dc505b7cbbd234f8f94165e2696",
        "sha256": "06k21wbyhhvq2f1xczszh3c2934p0m02by3l2ixvd6nkwrqklax7",
        "type": "tarball",
        "url": "https://github.com/NixOS/nixpkgs/archive/eb73405ecceb1dc505b7cbbd234f8f94165e2696.tar.gz",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
    }
}

nixpkgs is using branch release-20.03 while the documentation is saying "Currently, you would be tracking the
release-21.05 branch."

[Slim]

I also encountered this problem, which prevented me from hitting the haskell.nix binary cache because 20.03 is not a supported nixpkgs pinning.