nixos/spamassassin: Fix network requirement on boot
nixos/spamassassin: Fix network requirement on boot

sa-update currently runs as part of the pre-start script of spamd. The
network is not guaranteed to be online at that point and even if we
were to depend on that, it makes the bootup brittle, as there is a
reliance on SpamAssassin's update server as a startup dependency on
boot.

Refactor the setup to move the pre-start script into its own unit.
This allows to perform the setup task only once. Continuous updates
are already done by sa-update.service triggered by sa-update.timer.
Only run sa-update in case /var/lib/spamassassin is empty.

While we are on it, let sa-update.service depend on the network being
online.

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