nixos-rebuild: use sudo when whoami != root
Currently, executing `nixos-rebuild switch` as a non-root user will
result in a somewhat confusing error about being unable to link a
profile to a nix store path. This is not ideal, especially as we already
have most of the code to handle this properly and use `sudo` to elevate
permissions to install.
This is preferrable for flakes (better eval caching), and also more
intuitive for new users.
