feat: resolve rococo-runtime error

[th7nder]

Description

There was an issue.

---- real_world_use_case stdout ----
thread 'real_world_use_case' panicked at maat/tests/real_world.rs:290:63:
called `Result::unwrap()` on an `Err` value: GeneratorError(ProviderError(FileGenerationFailed(Error: 
   0: Invalid input: Rococo development wasm not available

Now it's gone.

Important points for reviewers

That's because in Nix supplied polkadot binary, SKIP_WASM_BUILD=1 is disabled.
Needed to generate chainspec by directly downloading polkadot binary (https://github.com/paritytech/polkadot-sdk/releases/download/polkadot-stable2412-1/polkadot) and running /polkadot build-spec --chain rococo-local > rococo-local.json. We won't need to add chain_spec_path` when paritytech/polkadot-sdk#7008 is merged and nix updated.

Additionally,

• we now follow https://github.com/andresilva/polkadot.nix/tree/master instead of nixpkgs/nixos-unstable, as it's the source of truth for it anyways and gets updated faster.

Checklist

• Are there important points that reviewers should know?
  • If yes, which ones?
• Make sure that you described what this change does.
• If there are follow-ups, have you created issues for them?
  • If yes, which ones? / List them here
    • Get rid of rococo-local.json #718
• Have you tested this solution?