disallow aliases in the nixpkgs-tarball eval

this allows ofborg to still check the aliases.nix file for errors
unlike https://github.com/NixOS/ofborg/pull/594 which would have
disabled aliases in outpaths.nix

errors in the alias.nix file are caught by outpaths.nix eval so we can
disable aliases here

ofborg/src/tasks/eval/nixpkgs.rs

@@ -529,6 +529,9 @@ impl<'a> EvaluationStrategy for NixpkgsStrategy<'a> {
                     String::from("--arg"),
                     String::from("nixpkgs"),
                     nixpkgs_arg_value.clone(),
+                    String::from("--arg"),
+                    String::from("config"),
+                    String::from("{ allowAliases = false; }"),
                     String::from("./pkgs/top-level/release.nix"),
                     String::from("-A"),
                     String::from("tarball"),