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

Created by  Maximilian Bosch  on September 26, 2020
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