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
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