Stop explicitly installing Ubuntu's python package, since most of the
Python 2 packages (and importantly pip/setuptools) have been removed in
this Ubuntu version.
The general skeleton of each job is the same, so consolidate the
definition to a single job with an explicit matrix.
Use conditional steps to handle steps that are unique to a certain job
(e.g., installing clang-11 for better ASAN support).
The env var is being set to the default value, as of ccache 3.3, which
is available in Zesty and newer. All of our CI is using Bionic, so this
is already available.
