Use nixUnstable

Dependencies

• [2] LCRBIWH3 Fix building on Nixpkgs 17.09
• [3] QPDG4NEU release.nix: Use fetchGit to get Nixpkgs
• [4] 6CZHII3Q Use <...> syntax
• [5] 6K5PBUUN Use buildEnv to combine Hydra's Perl dependencies
• [6] X7IAG7JT Simplify running nix-shell
• [7] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."
• [8] GGWH5GY7 Remove unused dependency on aws-sdk-cpp
• [9] GUOCMPZ2 Use Nix 2.0 and default GCC
• [10] T4LLYESZ * Nix expression for building Hydra.
• [11] YQWH4POV * Simplify.
• [12] C7CXMZ66 hydra: do not perform git clone every time. in stead work on local clone and pull
• [13] UBPWXGKF Move build logic for the core hydra package into its own default.nix.

Change contents

• replacement in release.nix at line 1
  [4.1]→[4.250:315](∅→∅)
  − { hydraSrc ? { outPath = ./.; revCount = 1234; rev = "abcdef"; }
  [4.1]

  [3.0]
  + { hydraSrc ? builtins.fetchGit ./.
• replacement in release.nix at line 45
  [4.1188]→[2.0:41](∅→∅)
  − nix = pkgs.nixStable2 or pkgs.nix;
  [4.1188]

  [4.731]
  + nix = pkgs.nixUnstable or pkgs.nix;