Update the nix build configuration. (#13706)

* Update the nix build configuration.

Remove the old shell.nix with some legacy versions pinned and replace it with a flake-based shell. It installs rust via rustup instead of fenix to be more generally compatible with the guidelines.

This also adds the rust-toolchain.toml spec with all the components required for wasm, and everything else to make rust-analyzer & clippy happy.

* Also add the top level flake for hacking on the substrate as whole

* Remove the envrc and ignore it instead.

* Remove the top-level configuration

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Co-authored-by: parity-processbot <>