Embrace Nix 2.4+

What's Changed

• By using nix 2.4 features, we can finally allow shellHook to be run even for cached shells and no longer cache environment variables by the user, thanks to @bbenne10 in #155 and @typetetris (#153)
• Convert ci tests to nix flake app test-runner by @typetetris in #154

New Contributors

• @typetetris made their first contribution in #154

Full Changelog: 1.6.1...2.0.0