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.
(cherry picked from commit bfab392e6e)
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
