Fix build with latest Nix

Recently a few internal APIs have changed[1]. The `outputPaths` function
has been removed and a lot of data structures are modeled with
`std::optional` which broke compilation.

This patch updates the code in `hydra-queue-runner` accordingly to make
sure that Hydra compiles again.

[1] https://github.com/NixOS/nix/pull/3883

src/hydra-eval-jobs/hydra-eval-jobs.cc

@@ -1,5 +1,6 @@
 #include <map>
 #include <iostream>
+#include <thread>
 
 #include "shared.hh"
 #include "store-api.hh"