Simple nix setup

[gilligan]

Overview

This PR introduces a simple Nix setup that works on Linux and macOS and evaluates quickly. The key changes are:

• Very small flake.nix with minimal dependencies
• No nix builds, the flake only provides a development environment
• No more spago2nix and npmlock2nix, builds happen with the usual build tools in a nix shell
• The ci.yaml just invokes make for onchain and offchain
• Simplified raw-scripts check to verify if raw-scripts are up-to-date
• spago has been updated to a recent version and installed via npm

Prereview checklist

• Changes have been documented by running changie new
• All tests pass in CI
• PR was self-reviewed

[kpinter-iohk]

💥