talkplugin: Install in a separate task

This is kind of a mess. google-chrome-stable installs the talkplugin
repository, which we previously installed ourselves, but now we do not
because the duplicate in google-chrome-stable results in benign errors
from apt. This means that we now need to separate out the installation
of google-chrome-stable and google-talkplugin, because
google-talkplugin cannot be found until after the installation of
google-chrome-stable is complete.