Add pyproject-nix overlay

[juliankuners]

Previously, when RV nix derivations were using poetry2nix, overriding a nix input in nix was also overriding the python code and resources used in poetry2nix. This allowed for easy overrides of python and K semantics changes by utilizing nix overrides.

With the migration to uv and uv2nix, this was behavior was removed due to the respective code not documenting it's purpose. In addition, due to uv2nix relying on state-of-the-art python nix packaging tooling pyproject-nix instead of nixpkgs, ensuring identical override behaviour requires more nix logic.

This PR re-introduces this kind of behaviour with uv2nix, though it depends on newly introduced overlays in the k nix flake.

Example

You can override k by running: kup install kevm --override k-framework ~/path/to/local-k-checkout. This will now also build kevm with the python and K code in ~/path/to/local-k-checkout.

[juliankuners]

K releases are currently blocked. As this PR depends on recently introduced nix overlays in K, this PR will fail until a new release of K was published and merged into evm-semantics.