Point the "mail" command to use the system-wide mail.rc

As per the info page, the GNU "mail" command is supposed to default to SYSCONFDIR/mail.rc but this path doesn't make sense. Hardcode this to the systemwide path (/etc). This allows tools such as cron to make use of aliases.

pkgs/tools/networking/mailutils/default.nix

@@ -47,6 +47,7 @@ stdenv.mkDerivation rec {
     "--with-gsasl"
     "--with-mysql"
     "--with-path-sendmail=${system-sendmail}/bin/sendmail"
+    "--with-mail-rc=/etc/mail.rc"
   ];
 
   readmsg-tests = let