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

This reverts commit c2e2b42877, which
breaks building with nixUnstable.

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

@@ -275,7 +275,7 @@ int main(int argc, char * * argv)
 
                 /* Prevent access to paths outside of the Nix search path and
                    to the environment. */
-                settings.restrictEval = true;
+                evalSettings.restrictEval = true;
 
                 if (releaseExpr == "") throw UsageError("no expression specified");