release: bump to 19.03

Dependencies

• [2] FJ5H26J6 Fix building against nix master
• [3] P33UMMOO Use nixUnstable
• [4] T4LLYESZ * Nix expression for building Hydra.
• [5] YQWH4POV * Simplify.
• [6] QPDG4NEU release.nix: Use fetchGit to get Nixpkgs
• [7] BGMBEXS6 Gradually increase maxHeapSize
• [*] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."

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-18.09-small"; }
  [3.35]

  [3.65]
  + , nixpkgs ? builtins.fetchGit { url = https://github.com/NixOS/nixpkgs-channels.git; ref = "nixos-19.03-small"; }
• edit in release.nix at line 139
  [9.3433]

  [9.3433]
  +

  + NIX_LDFLAGS = [

  + "-lpthread"

  + ];