hydra: upgrade nixpkgs to 19.09

Dependencies

• [2] AVGER75N release: bump to 19.03
• [3] T4LLYESZ * Nix expression for building Hydra.
• [4] FJ5H26J6 Fix building against nix master
• [5] BGMBEXS6 Gradually increase maxHeapSize
• [6] P33UMMOO Use nixUnstable
• [7] QPDG4NEU release.nix: Use fetchGit to get Nixpkgs
• [8] YQWH4POV * Simplify.

Change contents

• replacement in release.nix at line 2
  [3.35]→[2.0:114](∅→∅)
  − , nixpkgs ? builtins.fetchGit { url = https://github.com/NixOS/nixpkgs-channels.git; ref = "nixos-19.03-small"; }
  [3.35]

  [3.65]
  + , nixpkgs ? builtins.fetchGit { url = https://github.com/NixOS/nixpkgs-channels.git; ref = "nixos-19.09-small"; }