Rollup merge of #129620 - WaffleLapkin:flake, r=Noratrieb

Provide a more convinient way of developing rustc on NixOS

This PR adds envrc files which, once symlinked as  `.envrc` will activates a dev shell from `src/tools/nix-dev-shell/flake.nix` or `src/tools/nix-dev-shell/shell.nix`.

This is based on
- [Current rustc dev guide recommendation for NixOS](https://rustc-dev-guide.rust-lang.org/building/suggested.html?highlight=nix#using-nix-shell)
- https://github.com/oxalica/rust-overlay?tab=readme-ov-file#use-in-devshell-for-nix-develop
- [Nora's `x` nix package](https://github.com/Noratrieb/nixos/tree/26ea68e1a0aadaab313c1b5a8c1033a9770bd138/custom-pkgs/x)
- https://github.com/rust-lang/rustup/pull/2891
- [Direnv: use flake/nix according to availability](https://discourse.nixos.org/t/direnv-use-flake-nix-according-to-availability/29825)

This is something that I plan to use personally, but I thought it might be worth upstreaming :)

r? Noratrieb