.devcontainer: add TODO for nixfmt-rfc-style alias

The alias should be dropped only once `pkgs.nixfmt` is unlikely to be
nixfmt-classic.

I.e. after nixfmt 1.0 is in a stable release, but before the
nixfmt-rfc-style alias starts warning.

See https://github.com/NixOS/nixpkgs/issues/425583

Changed files
+2
.devcontainer
+2
.devcontainer/devcontainer.json
···
"ghcr.io/devcontainers/features/nix:1": {
// fails in the devcontainer sandbox, enable sandbox via config instead
"multiUser": false,
+
// TODO: nixfmt-rfc-style → nixfmt (once it's in a stable release)
+
// https://github.com/NixOS/nixpkgs/issues/425583
"packages": "nixpkgs.nixd,nixpkgs.nixfmt-rfc-style",
"useAttributePath": true,
"extraNixConfig": "experimental-features = nix-command flakes,sandbox = true"