Install the default postgresql version for the distribution.

Summary:
If running this command on bullseye, there may already be a postgresql-12-server
installed; so pytest-postgresql will try to use postgresql-12's pg_ctl, which
does not exist.

Using the virtual package 'postgresql' instead of 'postgresql-11' installs
the right version for the distribution.

Reviewers: Reviewers, olasd

Differential Revision: https://forge.softwareheritage.org/D3631