buildEnv: allowSubstitutes = false

trivial derivation

pkgs/build-support/buildenv/default.nix

@@ -63,6 +63,7 @@ runCommand name
       priority = drv.meta.priority or 5;
     }) paths);
     preferLocalBuild = true;
+    allowSubstitutes = false;
     # XXX: The size is somewhat arbitrary
     passAsFile = if builtins.stringLength pkgs >= 128*1024 then [ "pkgs" ] else null;
   }