nixos/spamassassin: set home for spamd user

This should hopefully fix errors like the following:

plugin: eval failed: bayes: (in learn) locker: safe_lock: cannot create tmp lockfile /var/empty/.spamassassin/bayes.lock.atuin.qyliss.net.398779 for /var/empty/.spamassassin/bayes.lock: No such file or directory

Changed files
+1
nixos
modules
services
+1
nixos/modules/services/mail/spamassassin.nix
···
users.users.spamd = {
description = "Spam Assassin Daemon";
+
home = "/var/lib/spamassassin";
uid = config.ids.uids.spamd;
group = "spamd";
};