Fix broken build.x86_64-linux job (#573)

The job has been failing since https://hydra.nixos.org/eval/1461286 with the following error:

hydra-eval-jobs.cc:278:17: error: 'evalSettings' was not declared in this scope
                 evalSettings.restrictEval = true;
                 ^~~~~~~~~~~~

This is likely due to a typo in 0882519 where that line and the corresponding comment were moved, and settings was changed in that one place to evalSettings.

I reproduced the error by running nix-build release.nix -A build.x86_64-linux on my machine, and this small change fixes it.

Created by  Jacob Mitchell  on July 7, 2018
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