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