add shell and flake for nix-shell or nix develop for Rust

dev/rust/flake.nix

@@ -0,0 +1,21 @@
+{
+  description = "A very basic flake";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+  let
+    pkgs = nixpkgs.legacyPackages."x86_64-linux";
+  in
+  {
+
+    devShells."x86_64-linux".default = pkgs.mkShell {
+      packages = with pkgs; [ cargo rustc ];
+    };
+
+    RUST_BACKTRACE = 1;
+
+  };
+}