runDhallCommand: append extra nativeBuildInputs

lib/default.nix

@@ -18,8 +18,9 @@ let
 in rec {
 
   runDhallCommand = name: env: script:
-    nixpkgs.runCommand name
-    ({ nativeBuildInputs = [ buildPackages.dhall ]; } // env) ''
+    nixpkgs.runCommand name (env // {
+      nativeBuildInputs = [ buildPackages.dhall ] ++ env.buildPackages or [ ];
+    }) ''
       ${dhallCachePrelude}
       ${script}
     '';