🧑‍💻 feat: add cargo-watch to nix dev shell (#452)

- Contributors using NixOS won't need to install cargo-watch
  imperatively via `cargo` since it is already packaged up in `nixpkgs`

flake.nix

@@ -39,6 +39,7 @@
             nodePackages_latest.stylelint
             redis
             rustPackages.clippy
+            cargo-watch
             rustc
             yamllint
             openssl