Adjust to the NixOS/Nixpkgs merge

Dependencies

• [2] A3A2O37I Work around 9P corruption on 32-bit
• [3] UGA45FNC Add a plugin for backing up builds in s3
• [4] 6CZHII3Q Use <...> syntax
• [5] 4UBEIJRA release.nix: Remove `services' arguments to ${nixos}/lib/testing.nix.
• [6] LZVO64YG Merge in the first bits of the API work
• [7] YQWH4POV * Simplify.
• [8] 7C6HSXUQ * Start of a regression test for Hydra.
• [9] IZYR3KTL release.nix: Get rid of the "system" argument
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 152
  [4.238]→[4.156:217](∅→∅),[4.162]→[4.156:217](∅→∅)
  − with import <nixos/lib/testing.nix> { inherit system; };
  [4.238]

  [4.239]
  + with import <nixpkgs/nixos/lib/testing.nix> { inherit system; };
• replacement in release.nix at line 180
  [4.65]→[4.65:126](∅→∅)
  − with import <nixos/lib/testing.nix> { inherit system; };
  [4.65]

  [2.0]
  + with import <nixpkgs/nixos/lib/testing.nix> { inherit system; };
• replacement in release.nix at line 218
  [3.101]→[3.101:162](∅→∅)
  − with import <nixos/lib/testing.nix> { inherit system; };
  [3.101]

  [3.162]
  + with import <nixpkgs/nixos/lib/testing.nix> { inherit system; };