Add Nix flake build

[Baltoli]

Part of https://github.com/runtimeverification/mx-backend/issues/95; we currently don't have any Nix setup for this repo and we will need to flakify it in order to get kup installation working.

[Baltoli]

Documenting my workflow for getting this working from scratch:

• Basing my starting point on the MIR semantics flake, which is a useful reference because it's new, maintained, and reasonably self-contained (in contrast to something like KEVM, which is much bigger and more complex).
• First look - the flake will end up looking a bit different to KMIR because the main entry point isn't a Poetry app, but rather a shell script. We will end up using poetry2nix though, for the format conversion (?) tool.
• Inputs to flake can be pretty straightforwardly copied across from KMIR; I've left out a Nixpkgs divergence between Pyk + the rest of the toolchain as it's not yet clear why this is needed. Could be templated similarly to nix flake init.
• Next step is to build a trivial empty kwasm derivation that depends on K. Newer semantics will do this using poetry2nix, but we've got a shell script so need to do things a bit manually.