release.nix: Use fetchGit to get Nixpkgs

Dependencies

• [2] ZXVPPARR Cleanup
• [3] X7IAG7JT Simplify running nix-shell
• [4] WOYOIGB7 Disable 32-bit builds again
• [5] A3A2O37I Work around 9P corruption on 32-bit
• [6] UGA45FNC Add a plugin for backing up builds in s3
• [7] F6IBTEIB Disable the S3 backup test
• [8] T4LLYESZ * Nix expression for building Hydra.
• [9] YQWH4POV * Simplify.
• [10] XWNMTW6P Drop building on i686-linux
• [11] LZVO64YG Merge in the first bits of the API work
• [12] WQ2VQ7H3 Use hydra-module.nix in the tests
• [13] 2GYW62QS Allow running on i686-linux
• [14] WGDX4QRI Adjust to the NixOS/Nixpkgs merge
• [15] Z5OWQ4KZ Check Nix version
• [16] IZYR3KTL release.nix: Get rid of the "system" argument
• [17] 6CZHII3Q Use <...> syntax
• [18] 7C6HSXUQ * Start of a regression test for Hydra.

Change contents

• edit in release.nix at line 2
  [3.315]

  [4.65]
  + , nixpkgs ? builtins.fetchGit { url = https://github.com/NixOS/nixpkgs-channels.git; ref = "nixos-18.03"; }
• replacement in release.nix at line 7
  [4.1]→[4.1:28](∅→∅)
  − with import <nixpkgs/lib>;
  [4.1]

  [4.0]
  + with import (nixpkgs + "/lib");
• replacement in release.nix at line 11
  [4.8]→[4.6:36](∅→∅)
  − pkgs = import <nixpkgs> {};
  [4.8]

  [4.36]
  + pkgs = import nixpkgs {};
• replacement in release.nix at line 41
  [4.738]→[4.144:191](∅→∅)
  − with import <nixpkgs> { inherit system; };
  [4.738]

  [4.738]
  + with import nixpkgs { inherit system; };
• replacement in release.nix at line 174
  [4.238]→[4.0:69](∅→∅)
  − with import <nixpkgs/nixos/lib/testing.nix> { inherit system; };
  [4.238]

  [4.307]
  + with import (nixpkgs + "/nixos/lib/testing.nix") { inherit system; };
• replacement in release.nix at line 189
  [4.65]→[4.70:139](∅→∅)
  − with import <nixpkgs/nixos/lib/testing.nix> { inherit system; };
  [4.65]

  [4.194]
  + with import (nixpkgs + "/nixos/lib/testing.nix") { inherit system; };
• replacement in release.nix at line 218
  [4.101]→[4.140:209](∅→∅)
  − with import <nixpkgs/nixos/lib/testing.nix> { inherit system; };
  [4.101]

  [2.92]
  + with import (nixpkgs + "/nixos/lib/testing.nix") { inherit system; };