Detect an existing Nix install in bootstrap.sh

Detect an existing Nix install in bootstrap.sh bootstrap.sh runs under bash and never reads .zshrc, so `command -v nix` failed even with Nix installed. Step 2 then re-ran the installer, which reported an already-completed plan and suggested uninstalling. Sources the Nix daemon profile up front so every later check sees it, and reports "already installed" rather than reinstalling. The sourcing is guarded on two counts: an `&&` chain would return non-zero and abort under `set -e` on a machine with no /nix at all, and the profile script is not written against `set -u`. Both paths were tested under a bare `env -i` bash.