nixos/postfix: make symlink /var/mail -> /var/spool/mail

This solves the problem of e.g. mutt not finding mail unless the user
sets MAIL=/var/spool/mail/$USER.

The default MAIL variable seems come from bash. Reasons for adding
symlink instead of changing MAIL default in bash:

- No need to rebuild world
- FHS recommends /var/mail over /var/spool/mail anyway[1]. Better fix
  NixOS mail location than change MAIL in bash to something that doesn't
  work on non-NixOS (however unlikely that users run nixpkgs bash on a
  non-NixOS distro...).

[1] http://www.pathname.com/fhs/pub/fhs-2.3.html#VARMAILUSERMAILBOXFILES