Adds cachix to CI and flake (like hyberbole)

[Skyfold]

There is no rush on this PR, you can look at this after your vacation. This mirrors the changes in hyperbole:

Updates

• cabal.project includes examples, so just like hyperbole there is only one project with two cabal files
• nix flake check also checks nix develop can run cabal --dry-run repl
• GitHub CI uploads the build dependencies to cachix
• adds cachix to flake so users can benefit from CI builds
• runs CI on all branches and all pull requests
  • Only pushes will update the cache since pull requests do not get access to secrets

Requires

• need to add CACHIX_AUTH_TOKEN as a repository secret
  • It can be found in: settings -> secrets and variables -> Actions -> New repository secret
  • you need to create a cachix account before I can add you (they don't have an invite system)
  • To create the auth token: Settings -> Auth Tokens -> Generate
    • Make sure it is (Write + Read), and (Never) expires

You can merge this without creating a cachix account and setting CACHIX_AUTH_TOKEN since my fork already populated the cache.

[Skyfold]

The macOS build is failing. I'll let you know when it is fixed so this can be merged.

[Skyfold]

This is done. Once this is merged I'll put up the hyperbole PR using this as the web-view input.

[Skyfold]

Notes:

1. Adds macOS support to the workflow
2. creates a separate overriddenHaskellPackages to workaround this isssue
3. Removes warp references in binary as explained here
4. updates README.md to better explain nix usage.