Patch nixpkgs input for lib test suite

The `lib` test suite has a few test cases that check the output of
`nix-instantiate --eval-only`. I don't think that output is guaranteed by
any compatibility policy, so I think we're good to change it, but we do
need to patch the tests. Once we have the output finalized here, we can
merge this PR in nixpkgs and then update the Flake input reference and
remove this patch.

flake.nix

@@ -11,7 +11,7 @@
   # Once this is updated to something containing
   # https://github.com/NixOS/nixpkgs/pull/271423, don't forget
   # to remove the `nix.checkAllErrors = false;` line in the tests.
-  inputs.nixpkgs.url = "github:NixOS/nixpkgs/staging-23.05";
+  inputs.nixpkgs.url = "github:9999years/nixpkgs/update-tests-for-nix-9606-staging-23.05";
   inputs.nixpkgs-regression.url = "github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2";
   inputs.lowdown-src = { url = "github:kristapsdz/lowdown"; flake = false; };
   inputs.flake-compat = { url = "github:edolstra/flake-compat"; flake = false; };