nixos(spamassassin): provide /etc/spamassassin to fix sa-learn et al

Spamassassin expects its system-wide configuration at /etc/spamassassin, and
some user tools (like sa-learn) need to read those configuration files.
Therefore, we provide a symlink from /etc/spamassassin to the appropriate Nix
store path to make sure those tools work without the user having to pass an
elaborate --siteconfig path that, potentially, changes every time the system
updates.

Fixes https://github.com/NixOS/nixpkgs/issues/29414.