Refactor the Nix packaging for reduced duplication and to provide a more useful shell environment

This changes default.nix, tests.nix, and shell.nix so that we don't need to duplicate default.nix's parameters anywhere yet we can still pass them through from everywhere.

It also adds the lint tools to the environment defined by shell.nix for easier interactive linting during development.

It increases the version of Nix required to 2.5 (maybe 2.4 works but I don't see an easy way to get 2.4 to test it out so let's just say 2.5) for shell.nix and tests.nix but not for default.nix.