Refactor Nix sources

[lucperkins]

This PR refactors the Nix sources in a few ways:

• It removes the naersk dependency from the main cargo-pgx package build
• It uses Nixpkgs' buildRustPackage for the cargo-pgx build
• It brings things in line with some flake best practices and silences some errors (e.g. using package.default instead of defaultPackage)
• It makes the cargo-pgx package buildable on macOS via some optional buildInputs
• It introduces flake-utils helpers for system-specific needs

An important thing to note is that I've left the lib output as-is, as I'm not fully sure what the current intentions are for that.

[Hoverbear]

Without affecting the lib or providing a sample of how to build a pgx extension using buildRustPackage this unfortunately has pretty limited use. :(

[workingjubilee]

Thank you for offering but no.