fetchTarball {
  url = "https://github.com/NixOS/nixpkgs/archive/b0bbacb52134a7e731e549f4c0a7a2a39ca6b481.tar.gz";
  sha256 = "15ix4spjpdm6wni28camzjsmhz0gzk3cxhpsk035952plwdxhb67";
}

fetchTarball {
  url = "https://github.com/NixOS/nixpkgs/archive/9beb0d1ac2ebd6063efbdc4d3631f8ce137bbf90.tar.gz";
  sha256 = "1v95779di35qhrz70p2v27kmwm09h8pgh74i1wc72v0zp3bdrf50";
}

Upgrade to latest commit in nixos-unstable channel
Upgrade to latest commit in nixos-unstable channel

  gcc     8.3.0  -> 9.2.0
  python3 3.7.5  -> 3.7.6
  sqlite  3.30.0 -> 3.30.1
