Move most of AddBuilds to hydra-eval-jobset

Having this stuff in a separate module is a remnant of the time when
hydra-server could add builds to the queue directly. This is no longer
the case.

src/lib/Hydra/Helper/Nix.pm

@@ -10,6 +10,7 @@ use Hydra::Model::DB;
 use Nix::Store;
 use Encode;
 use Sys::Hostname::Long;
+use IPC::Run;
 
 our @ISA = qw(Exporter);
 our @EXPORT = qw(