configuration.nix

{ pkgs, ... }:

{
  service.hydra.package = with pkgs; hydra.overrideAttrs (super: {
    patches = [ (fetchpatch {
      url = "https://gist.githubusercontent.com/yegortimoshenko/6d78a4fbc7b5b21234d298b3ce53ea68/raw/4193a6e9b280582a1b1265095734ece333d6719b/hydra-no-restrict-eval.diff";
      sha256 = "16gwlsr6azwxj9q0b38kkb52xwc2h7ncahymdkdcrm8046naxj6k";
    }) ];
  });
}

hydra-no-restrict-eval.diff

diff --git a/src/hydra-eval-jobs/hydra-eval-jobs.cc b/src/hydra-eval-jobs/hydra-eval-jobs.cc
index ce6967b5..0f3abc8f 100644
--- a/src/hydra-eval-jobs/hydra-eval-jobs.cc
+++ b/src/hydra-eval-jobs/hydra-eval-jobs.cc
@@ -293,7 +293,7 @@ int main(int argc, char * * argv)
 
                 /* Prevent access to paths outside of the Nix search path and
                    to the environment. */
-                evalSettings.restrictEval = true;
+                evalSettings.restrictEval = false;
 
                 if (releaseExpr == "") throw UsageError("no expression specified");