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

---------

Co-authored-by: parity-processbot <>

substrate/bin/node-template/flake.nix

@@ -0,0 +1,22 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }: flake-utils.lib.eachDefaultSystem (system:
+    let
+      pkgs = import nixpkgs { inherit system; };
+    in
+    {
+      devShells.default = pkgs.mkShell {
+        packages = with pkgs; [
+          rustup
+          clang
+          protobuf
+        ];
+
+        LIBCLANG_PATH = "${pkgs.libclang.lib}/lib";
+      };
+    });
+}