shell.nix: use Rust overlay

shell.nix

@@ -1,18 +1,25 @@
+# <shell.nix>
 { pkgs ? import <nixpkgs> {}}:
 
+let
+  rust_overlay = import (builtins.fetchTarball "https://github.com/oxalica/rust-overlay/archive/master.tar.gz");
+  pkgs = import <nixpkgs> { overlays = [ rust_overlay ]; };
+  ruststable = (pkgs.rust-bin.stable.latest.default.override {
+    extensions = [
+      "rust-src"
+    ];
+  });
+in
 pkgs.mkShell {
   buildInputs = with pkgs; [
-    rustc
-    cargo
-    rustfmt
+    ruststable
     rust-analyzer
-    clippy
+
+    bacon
 
     bacon
 
     pkg-config
     openssl
   ];
-
-  RUST_BACKTRACE = 1;
 }