default.nix

let
  sources = import ./nix/sources.nix { };
  nixpkgs = import sources.nixpkgs { };
  nixpkgs-ruby267 = import sources.nixpkgs-ruby267 { };
in nixpkgs.mkShell {
  buildInputs = [
    nixpkgs.go
    nixpkgs-ruby267.ruby_2_6
  ];
}

nix-sources.json

{
    "nixpkgs": {
        "branch": "release-21.05",
        "description": "Nix Packages collection",
        "homepage": "",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "5f244caea76105b63d826911b2a1563d33ff1cdc",
        "sha256": "1xlgynfw9svy7nvh9nkxsxdzncv9hg99gbvbwv3gmrhmzc3sar75",
        "type": "tarball",
        "url": "https://github.com/NixOS/nixpkgs/archive/5f244caea76105b63d826911b2a1563d33ff1cdc.tar.gz",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
    },
    "nixpkgs-ruby267": {
        "branch": "nixpkgs-unstable",
        "description": "Nix Packages collection",
        "homepage": "",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "860b56be91fb874d48e23a950815969a7b832fbc",
        "sha256": "07i03028w3iak0brdnkp79ci8vqqbrgr5p5i9sk87fhbg3656xhw",
        "type": "tarball",
        "url": "https://github.com/NixOS/nixpkgs/archive/860b56be91fb874d48e23a950815969a7b832fbc.tar.gz",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
    }
}

output

$ go version
go version go1.16.4 darwin/amd64

$ ruby --version
ruby 2.6.7p197 (2021-04-05) [x86_64-darwin17]

Shouldn't there be import ./nix/sources.json?

No, nix/sources.nix is the file autogenerated by niv which contains a nix expression that loads nix/sources.json.

Can you show complete working example?