nix: fix broken flake build, drop legacy flake-compat, support non-AVX2 CPUs

[mirdukkkkk]

nix build/nix run are currently broken on main and can't produce a package at all:

1. Cargo.lock has two git-sourced crates (apple-codesign, isideload) with no outputHashes. buildRustPackage's cargoLock.lockFile mode requires an explicit hash for any git dependency, since Nix can't derive one on its own. Without it, evaluation fails immediately with No hash was found while vendoring the git dependency
2. bun.lock is stale against package.json. Several dependency ranges in package.json (react, i18next, tauri plugins, etc.) were bumped without regenerating the lockfile, so bun2nix's offline vendored install can't satisfy them and falls back to a network resolve that the Nix sandbox correctly refuses. Regenerated with bun install
3. default.nix/shell.nix are legacy flake-compat shims that don't even work in the current flake - they compat into a devShell/shellNix output that the flake never defines. They're dead weight that just adds another input to keep in sync for no benefit on a modern Nix install
4. Added a bun version overlay (before the bun2nix overlay) pinning bun to 1.4.1 on x86_64-linux, fetched directly from GitHub releases. nixpkgs currently ships bun 1.3.13, which is built against AVX2 and won't run on older/budget CPUs without a separate baseline build; bun dropped that requirement as of 1.4, so pinning forward is simpler than carrying a baseline variant

I've tested these changes in my Nix config, and everything works perfectly fine.

[mirdukkkkk]

https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/by-name/il/iloader/package.nix