Refactor to flake-parts and add NixOS tests that exercises dynamic derivation builds

[elpdt852]

Supersedes #32
Fixes #3

Highlights

• Changes github actions to be consistent with nix-snapshotter
• Migrate to flake-parts in a way that's consistent with nix-snapshotter
• Adds NixOS tests that always exercises the derivation builds

Tests

I thought about the way #32 approached it, simply build it into a tmp store but it doesn't have the caching properties we'll get from magic-nix-cache. However, if we build it into /nix/store, it won't exercise the dynamic derivations part if it's cached. Using the nix-portable approach in #26 makes sense for development but we don't want to test nix-portable but actually a proper nix build.

I experimented whether we can run a NixOS test that:

1. Always exercises the dynamic derivations
2. Optimally caches everything
3. More flexibility in testing, e.g. running the built executable or inspecting it with ldd, etc.

Turns out it's possible and even older nixes should be able to run these NixOS tests as it uses a dynamic derivation enabled nix inside the VM.