nix-daemon: Start "nix-daemon" rather than "nix-worker --daemon"
Run the garbage collector as a systemd service

Running it from systemd rather than cron has several advantages:
systemd ensures that only one instance runs at a time; the GC can be
manually started/stopped; and logging goes to the journal.

We still need cron to start the service at the right time, but
hopefully soon we can get rid of cron entirely (once systemd supports
starting a unit at a specific time).