hydra: 2017-11-21 -> 2018-08-07

This bumps Hydra to the latest revision available. As Hydra doesn't have
a release model (and therefore no tags) ATM, the derivation will pin
against the actual git revision and the date of the commit in the
derivation name.

Additionally the following changes have been made:

* Dropped `postUnpack` phase. It is useful when working with the Hydra
  source (and no dirty changes shall be used in `release.nix`, but is has
  no use in `nixpkgs`).

* Added myself as maintainer to have more folks available in case of
  future breakage.

* Implemented support for Nix 2.0 and `unstable` (currently 2.1):

  Since 1672bcd230447f1ce0c3291950bdd9a662cee974 in NixOS/nix the
  evaluator differentiates between `settings` and `evalSettings`.
  Previously `restrictEval` in `hydra-eval-jobs.cc` has been set in
  `settings`, this doesn't work anymore in Nix 2.1 and is therefore
  incompatible to Nix 2.0 on an API level.

  To resolve this, the flag `isGreaterNix20` parses the version string
  of `pkgs.nix` and applies a patch if nix.version<=2.0.

  Furthermore the Hydra build with Nix 2.1 requires `boost` as build input
  which is not needed for Nix 2.0. To avoid unnecessary increase in the
  closure size this library will only used as build input for
  nix.version>2.0.

* Fixed the NixOS test for `hydra`:
  disabled binary cache to allow sandbox builds (otherwise it would
  query `cache.nixos.org` during the Hydra build inside the test).

  Additionally the trivial.nix jobset required simplification (as done
  in NixOS/hydra, e.g. tests/api-test.nix) as bash is not available in
  the build sandbox as builder (even when adding pkgs.bash to
  systemPackages).

  The easiest workaround to confirm a the functionality of a jobset
  without importing nixpkgs is to use the default shell /bin/sh which
  is mounted from `pkgs.busybox` into the build env
  (https://github.com/NixOS/nixpkgs/pull/44841#discussion_r209751972) in the
  VM and a named pipe to create $out.

Closes #44044