Gradually increase maxHeapSize

Dependencies

• [2] 5MP35ORV hydra-eval-jobs: Ugly hackery to reduce memory usage
• [3] P33UMMOO Use nixUnstable
• [4] QPDG4NEU release.nix: Use fetchGit to get Nixpkgs
• [5] T4LLYESZ * Nix expression for building Hydra.
• [6] X7IAG7JT Simplify running nix-shell
• [7] YQWH4POV * Simplify.
• [*] 4N5APGRG * Start of a helper tool to evaluate job expressions efficiently.

Change contents

• replacement in release.nix at line 2
  [3.35]→[4.0:108](∅→∅),[4.315]→[4.0:108](∅→∅)
  − , nixpkgs ? builtins.fetchGit { url = https://github.com/NixOS/nixpkgs-channels.git; ref = "nixos-18.03"; }
  [3.35]

  [4.65]
  + , nixpkgs ? builtins.fetchGit { url = https://github.com/NixOS/nixpkgs-channels.git; ref = "nixos-18.03-small"; }
• edit in src/hydra-eval-jobs/hydra-eval-jobs.cc at line 314
  [2.4036]

  [2.4036]
  +

  + maxHeapSize = maxHeapSize * 1.1;