perlPackages.MHonArc: fix installation (ZHF)

pkgs/top-level/perl-packages.nix

···
       sha256 = "1xmf26dfwr8achprc3n1pxgl0mkiyr6pf25wq3dqgzqkghrrsxa2";
     };
     propagatedBuildInputs = [ ];
+
+    installTargets = "install";
+
     meta = with stdenv.lib; {
       homepage    = http://dcssrv1.oit.uci.edu/indiv/ehood/mhonarch.html;
       description = "A mail-to-HTML converter";