hydra-eval-jobs: don't use restrict-eval for Flakes

Pure evaluation should cover all the uses for restrict-eval, and
restrict-eval currently breaks accessing Git inputs on Flakes[1].

[1]: NixOS/nix#7098

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

@@ -306,7 +306,9 @@ int main(int argc, char * * argv)
 
         /* Prevent access to paths outside of the Nix search path and
            to the environment. */
-        evalSettings.restrictEval = true;
+        if (!myArgs.flake) {
+            evalSettings.restrictEval = true;
+        }
 
         /* When building a flake, use pure evaluation (no access to
            'getEnv', 'currentSystem' etc. */