packaging: drop Disco (19.04) and add Focal (20.04)
Disco support ended in January 2020, and Focal does not have an announced EOL.
Something is now installing and configuring tzdata, which was throwing up an
interactive prompt to configure the timezone. Aside from being hostile to
automation, the prompt didn't actually accept input and hung the process. This
propagates the host's timezone into the image via environment variable in order
to skip the prompt, and avoid hardcoding a value.
Differential Revision: https://phab.mercurial-scm.org/D9396