tails::iso_builder: install pigz, for faster Vagrant box creation

pigz, i.e. "Parallel Implementation of GZip", takes advantage of multiple processors and multiple cores when compressing data.

Recent vagrant-libvirt's create_box.sh uses pigz if it's available, which speeds up the creation of our Vagrant build box on multi-core machines.

This is the Puppet equivalent of the corresponding change applied to our contribute/build doc page: tails@d606d47a