Fixes needed for CI with Ubuntu 20.04

[scpeters]

We have tested using Ubuntu 20.04 for gazebo9 and gazebo11 CI, but it failed in our first testing:

• gazebo11: https://build.osrfoundation.org/job/gazebo-ci-gazebo11-focal-amd64-gpu-nvidia/4/

• gazebo9: https://build.osrfoundation.org/job/gazebo-ci-gazebo9-focal-amd64-gpu-nvidia/7

[scpeters]

one issue is that check_test_ran.py should use python3 instead of python:

• https://github.com/osrf/gazebo/blob/gazebo11/tools/check_test_ran.py#L1

[scpeters]

from gazebo-tooling/release-tools#565 (comment)

we should use distro-specific packages.apt files like ign-physics:

• https://github.com/ignitionrobotics/ign-physics/blob/ign-physics5/.github/ci/packages-bionic.apt
• https://github.com/ignitionrobotics/ign-physics/blob/ign-physics5/.github/ci/packages-focal.apt

[scpeters]

20.04 builds fixed for gazebo11 in #3155

[scpeters]

fix backported to gazebo9 in #3278