Make nix-daemon.plist less fragile on macOS

We're calling `wait4path` on the full, resolved `@bindir@/nix-daemon` path.

That means we're hardcoding something like:

    /bin/wait4path /nix/store/zs9c5xhp3zv9p23qnjxp87nl5injsi1i-nix-2.3/bin/nix-daemon && /nix/var/nix/profiles/default/bin/nix-daemon

That seems unnecessarily fragile.

It might be better to wait4path on the path we intend to call.