hammer.py: postgres needs the most precise time zone when being set up

So on systems where America/Argentina/Ushuaia is available, it will not accept America/Argentina. This might be the case starting with Postgres 13 or it might have been here forever. Regardless we can write hammer to choose whichever time zone is most precise in /usr/share/zoneinfo.

See this for an example of the error: https://jenkins.aws.isc.org/view/All/job/packer/173/console

The same time zone worked for Alpine 3.12 with Postgres 12: https://jenkins.aws.isc.org/view/All/job/packer/168/console