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.lock

@@ -0,0 +1,43 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1678901627,
+        "narHash": "sha256-U02riOqrKKzwjsxc/400XnElV+UtPUQWpANPlyazjH0=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "93a2b84fc4b70d9e089d029deacc3583435c2ed6",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1679262748,
+        "narHash": "sha256-DQCrrAFrkxijC6haUzOC5ZoFqpcv/tg2WxnyW3np1Cc=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "60c1d71f2ba4c80178ec84523c2ca0801522e0a6",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}