hydra: 2020-03-24 -> 2020-04-07

Also removed `pkgs.hydra-flakes` since flake-support has been merged
into master[1]. Because of that, `pkgs.hydra-unstable` is now compiled
against `pkgs.nixFlakes` and currently requires a patch since Hydra's
master doesn't compile[2] atm.

[1] https://github.com/NixOS/hydra/pull/730
[2] https://github.com/NixOS/hydra/pull/732

pkgs/development/tools/misc/hydra/common.nix

@@ -4,7 +4,7 @@
 , guile, perl, postgresql, nukeReferences, git, boehmgc, nlohmann_json
 , docbook_xsl, openssh, gnused, coreutils, findutils, gzip, lzma, gnutar
 , rpm, dpkg, cdrkit, pixz, lib, boost, autoreconfHook, src ? null, version ? null
-, migration ? false
+, migration ? false, patches ? []
 }:
 
 with stdenv;
@@ -74,7 +74,7 @@ let
 in stdenv.mkDerivation rec {
   pname = "hydra";
 
-  inherit stdenv src version;
+  inherit stdenv src version patches;
 
   buildInputs =
     [ makeWrapper autoconf automake libtool unzip nukeReferences sqlite libpqxx