Skip nixos-rebuild when evaluated toplevel equals current system

To make a flake rebuild script skip no-op rebuilds, don't rely on git diff --cached --quiet: it misses pulled commits, flake.lock bumps and branch switches. Instead run nix eval --raw .#nixosConfigurations.$HOST.config.system.build.toplevel.outPath (after git add -A so the flake sees new files) and compare with readlink -f /run/current-system; equal means nothing to switch. Eval costs ~30s on a mid-size config but needs no sudo and builds nothing. Keep commit/push logic separate, since non-closure files (README, scripts) can change without changing the system.