nixpkgs has 23000 attributes, and our previous limit would be hit if you
have more than one nixpkgs in the environment, for example, because
`repl-overlays` will load the new stuff from the environment on top of
the existing environment.
This is not really testable since if we did write such a test, it would
just be testing this constant tbh...
Fixes: https://git.lix.systems/lix-project/lix/issues/337
Change-Id: I49197bfb4db55b082f914f0d70e84f5f5f110954